[Shell-escape] Escape toggle with \ prefix

E.g.
    check file.hs
will treat quote characters as literal characters, while
    \check file.hs
will assume quoting behavior

Backslash will be dropped, naturally.

src/GHCMod/Options/ShellParse.hs

@@ -36,4 +36,6 @@ go (c:cl) curarg accargs quotes
   | otherwise = go cl (c:curarg) accargs quotes
 
 parseCmdLine :: String -> [String]
-parseCmdLine comline = go comline [] [] False
+parseCmdLine ('\\':comline) = go comline [] [] False
+parseCmdLine [] = [""]
+parseCmdLine comline = words comline

test/ShellParseSpec.hs

@@ -8,11 +8,17 @@ import Test.Hspec
 spec :: Spec
 spec =
   describe "parseCmdLine" $ do
-    it "splits arguments" $
+    it "splits arguments" $ do
       parseCmdLine "test command line" `shouldBe` ["test", "command", "line"]
-    it "honors quoted segments" $
-      parseCmdLine "test command line \STXwith quoted segment\ETX"
+      parseCmdLine "\\test command line" `shouldBe` ["test", "command", "line"]
+    it "honors quoted segments if turned on" $
+      parseCmdLine "\\test command line \STXwith quoted segment\ETX"
         `shouldBe` ["test", "command", "line", "with quoted segment"]
-    it "squashes multiple spaces" $
+    it "doesn't honor quoted segments if turned off" $
+      parseCmdLine "test command line \STXwith quoted segment\ETX"
+        `shouldBe` words "test command line \STXwith quoted segment\ETX"
+    it "squashes multiple spaces" $ do
       parseCmdLine "test       command"
         `shouldBe` ["test", "command"]
+      parseCmdLine "\\test       command"
+        `shouldBe` ["test", "command"]