Fix #1369 (Use file extension to detect shell)

The precedence order that is used to determine the shell
is the following:
1. ShellCheck directive
2. Shebang
3. File extension
A new field `asFallbackShell` has been
added to the record type `AnalysisSpec`.

src/ShellCheck/Checker.hs

@@ -48,6 +48,15 @@ tokenToPosition startMap t = fromMaybe fail $ do
   where
     fail = error "Internal shellcheck error: id doesn't exist. Please report!"
 
+shellFromFilename filename = foldl mplus Nothing candidates
+  where
+    shellExtensions = [(".ksh", Ksh)
+                      ,(".sh", Sh)
+                      ,(".bash", Bash)
+                      ,(".dash", Dash)]
+    candidates =
+        map (\(ext,sh) -> if ext `isSuffixOf` filename then Just sh else Nothing) shellExtensions
+
 checkScript :: Monad m => SystemInterface m -> CheckSpec -> m CheckResult
 checkScript sys spec = do
     results <- checkScript (csScript spec)
@@ -69,6 +78,7 @@ checkScript sys spec = do
                 as {
                     asScript = root,
                     asShellType = csShellTypeOverride spec,
+                    asFallbackShell = shellFromFilename $ csFilename spec,
                     asCheckSourced = csCheckSourced spec,
                     asExecutionMode = Executed,
                     asTokenPositions = tokenPositions