Add option to highlight echoed recipe lines (#190)

Using bold and cyan, for visibility.

src/app.rs

@@ -115,6 +115,9 @@ pub fn app() {
     .arg(Arg::with_name("EVALUATE")
          .long("evaluate")
          .help("Prints evaluated variables"))
+    .arg(Arg::with_name("HIGHLIGHT")
+         .long("highlight")
+         .help("Highlight echoed recipe lines in bold"))
     .arg(Arg::with_name("JUSTFILE")
          .long("justfile")
          .takes_value(true)
@@ -338,6 +341,7 @@ pub fn app() {
   let options = RunOptions {
     dry_run:   matches.is_present("DRY-RUN"),
     evaluate:  matches.is_present("EVALUATE"),
+    highlight: matches.is_present("HIGHLIGHT"),
     overrides: overrides,
     quiet:     matches.is_present("QUIET"),
     shell:     matches.value_of("SHELL"),

src/lib.rs

@@ -424,8 +424,11 @@ impl<'a> Recipe<'a> {
         if options.dry_run
           || options.verbose
           || !((quiet_command ^ self.quiet) || options.quiet) {
-          warn!("{}", command);
+          let highlight = maybe_highlight(options.highlight
+                                          && options.use_color.should_color_stderr());
+          warn!("{}", highlight.paint(command));
         }
+
         if options.dry_run {
           continue;
         }
@@ -1031,6 +1034,14 @@ fn maybe_bold(colors: bool) -> ansi_term::Style {
   }
 }
 
+fn maybe_highlight(colors: bool) -> ansi_term::Style {
+  if colors {
+    ansi_term::Style::new().fg(ansi_term::Color::Cyan).bold()
+  } else {
+    ansi_term::Style::default()
+  }
+}
+
 impl<'a> Display for CompileError<'a> {
   fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
     use ErrorKind::*;
@@ -1143,6 +1154,7 @@ struct Justfile<'a> {
 struct RunOptions<'a> {
   dry_run:   bool,
   evaluate:  bool,
+  highlight: bool,
   overrides: Map<&'a str, &'a str>,
   quiet:     bool,
   shell:     Option<&'a str>,