allow setting default values for environment variables

This is useful for setting up a reasonable initial environment.
For example, on Windows you might want to set the `prompt` environment
so that some basic shell integration is enabled; this will cause new
tabs to open with the same cwd as the current tab:

```
set_environment_variables = { "prompt"="$E]7;file://localhost/$P$E\\$P$G" }
```

This setting is intended to apply only to the local domain.

refs: https://github.com/wez/wezterm/issues/146

pty/src/cmdbuilder.rs

@@ -227,21 +227,51 @@ impl CommandBuilder {
     /// adds/replaces the environment that was specified via the
     /// `env` methods.
     pub(crate) fn environment_block(&self) -> Vec<u16> {
+        // Holds an entry with its preferred key case; the environment
+        // has case insensitive variable names on windows, so we need
+        // to take care to avoid confusing things with conflicting
+        // entries, and we'd also like to preserve the original case.
+        struct Entry {
+            key: OsString,
+            value: OsString,
+        }
+
+        // Best-effort lowercase transformation of an os string
+        fn lowerkey(k: &OsStr) -> OsString {
+            if let Some(s) = k.to_str() {
+                s.to_lowercase().into()
+            } else {
+                k.to_os_string()
+            }
+        }
+
+        // Use a btreemap for a nicer sorted order if you review the
+        // environment via `set`.
+        let mut env_hash = std::collections::BTreeMap::new();
+
         // Take the current environment as the base
-        let mut env_hash: std::collections::HashMap<_, _> = std::env::vars_os().collect();
+        for (key, value) in std::env::vars_os() {
+            env_hash.insert(lowerkey(&key), Entry { key, value });
+        }
 
         // override with the specified values
-        for (k, v) in &self.envs {
-            env_hash.insert(k.to_owned(), v.to_owned());
+        for (key, value) in &self.envs {
+            env_hash.insert(
+                lowerkey(&key),
+                Entry {
+                    key: key.clone(),
+                    value: value.clone(),
+                },
+            );
         }
 
         // and now encode it as wide characters
         let mut block = vec![];
 
-        for (k, v) in env_hash {
-            block.extend(k.encode_wide());
+        for entry in env_hash.values() {
+            block.extend(entry.key.encode_wide());
             block.push(b'=' as u16);
-            block.extend(v.encode_wide());
+            block.extend(entry.value.encode_wide());
             block.push(0);
         }
         // and a final terminator for CreateProcessW

src/config/mod.rs

@@ -300,6 +300,12 @@ pub struct Config {
     /// as the positional arguments to that command.
     pub default_prog: Option<Vec<String>>,
 
+    /// Specifies a map of environment variables that should be set
+    /// when spawning commands in the local domain.
+    /// This is not used when working with remote domains.
+    #[serde(default)]
+    pub set_environment_variables: HashMap<String, String>,
+
     /// Specifies the height of a new window, expressed in character cells.
     #[serde(default = "default_initial_rows")]
     pub initial_rows: u16,
@@ -797,10 +803,17 @@ impl Config {
             }
         };
 
-        cmd.env("TERM", &self.term);
+        self.apply_cmd_defaults(&mut cmd);
 
         Ok(cmd)
     }
+
+    pub fn apply_cmd_defaults(&self, cmd: &mut CommandBuilder) {
+        for (k, v) in &self.set_environment_variables {
+            cmd.env(k, v);
+        }
+        cmd.env("TERM", &self.term);
+    }
 }
 
 fn default_ratelimit_line_prefetches_per_second() -> u32 {

src/mux/domain.rs

@@ -91,7 +91,10 @@ impl Domain for LocalDomain {
     ) -> Result<Rc<dyn Tab>, Error> {
         let config = configuration();
         let mut cmd = match command {
-            Some(c) => c,
+            Some(mut cmd) => {
+                config.apply_cmd_defaults(&mut cmd);
+                cmd
+            }
             None => config.build_prog(None)?,
         };
         if let Some(dir) = command_dir {