lib: doctests: add --verbose and --slow flags to executable

--slow turns off doctest's --fast flag, which skips reloading between
  tests.
--verbose shows progress output as tests are run, if doctest 0.16.0+
  is installed (and I believe will be harmless otherwise)

hledger-lib/package.yaml

@@ -156,6 +156,7 @@ tests:
     dependencies:
     - Glob >=0.7
     - doctest >=0.8
+    # doctest >=0.16.0 needed to show verbose progress output
     # doctest with ghc 8.4 on mac requires a workaround:
     # ~$ locate HSinteger-gmp-1.0.2.0.o
     # /Users/simon/.stack/programs/x86_64-osx/ghc-8.4.3/lib/ghc-8.4.2/integer-gmp-1.0.2.0/HSinteger-gmp-1.0.2.0.o

hledger-lib/tests/doctests.hs

@@ -1,14 +1,20 @@
 {-# LANGUAGE PackageImports #-}
 
 import Data.List
+import System.Environment
 import "Glob" System.FilePath.Glob
 import Test.DocTest
 
 main = do
+  args <- getArgs
   fs1 <- glob "Hledger/**/*.hs"
   fs2 <- glob "Text/**/*.hs"
   --fs3 <- glob "other/ledger-parse/**/*.hs"
   let fs = filter (not . isInfixOf "/.") $ ["Hledger.hs"] ++ fs1 ++ fs2
   doctest $ 
-    "--fast" :  -- https://github.com/sol/doctest#a-note-on-performance 
+    -- show verbose progress output
+    (if "--verbose" `elem` args then ("--verbose" :) else id) $ 
+    -- don't reload environment per test (opposite of doctest's --fast,
+    -- https://github.com/sol/doctest#a-note-on-performance)
+    (if "--slow" `elem` args then id else ("--fast" :)) $
     fs