Add option to look for sources in alternate root paths

Add a new optional flag "-r|--root ROOTPATHS", where ROOTPATHS is a
colon separated list of paths, that will look for external sources in
alternate roots.

This is particular useful when the run-time environment does not fully
match the development environment. The #shellcheck source=file directive
is useful, but has its limitations in certain scenarios. Also, in many
cases the directive could be removed from scripts when the root flag is
used.

Script example.bash:
  #!/bin/bash
  source /etc/foo/config

Example usage where etc/foo/config exists in skel/foo:
  # shellcheck -x -r skel/foo:skel/core example.bash

src/ShellCheck/Interface.hs

@@ -73,6 +73,8 @@ import qualified Data.Map as Map
 data SystemInterface m = SystemInterface {
     -- Read a file by filename, or return an error
     siReadFile :: String -> m (Either ErrorMessage String),
+    -- Find source file in alternate root paths
+    siFindSource :: String -> m (FilePath),
     -- Get the configuration file (name, contents) for a filename
     siGetConfig :: String -> m (Maybe (FilePath, String))
 }
@@ -287,6 +289,7 @@ data ColorOption =
 mockedSystemInterface :: [(String, String)] -> SystemInterface Identity
 mockedSystemInterface files = SystemInterface {
     siReadFile = rf,
+    siFindSource = fs,
     siGetConfig = const $ return Nothing
 }
   where
@@ -294,6 +297,7 @@ mockedSystemInterface files = SystemInterface {
         case filter ((== file) . fst) files of
             [] -> return $ Left "File not included in mock."
             [(_, contents)] -> return $ Right contents
+    fs file = return file
 
 mockRcFile rcfile mock = mock {
     siGetConfig = const . return $ Just (".shellcheckrc", rcfile)