Fix source maps for errors thrown from .coffee.md files

Before:

```
$ cat tmp.coffee.md
test

    a
$ ./bin/coffee tmp.coffee.md
ReferenceError: a is not defined
  at Object.<anonymous> (/src/coffee-script/tmp.coffee.md:2:3)
  ...
```

Note how the line and column numbers (2 and 3, respectively) are not
correct.

After:

```
$ ./bin/coffee tmp.coffee.md
ReferenceError: a is not defined
  at Object.<anonymous> (/home/lydell/forks/coffee-script/tmp.coffee.md:3:5)
  ...
```

Line 3, column 5 is the actual position of the `a` in tmp.coffee.md.

Supersedes and fixes #4204.

src/coffee-script.coffee

@@ -334,9 +334,11 @@ sourceMaps = {}
 # Generates the source map for a coffee file and stores it in the local cache variable.
 getSourceMap = (filename) ->
   return sourceMaps[filename] if sourceMaps[filename]
-  return unless path?.extname(filename) in exports.FILE_EXTENSIONS
-  answer = exports._compileFile filename, true
-  sourceMaps[filename] = answer.sourceMap
+  for ext in exports.FILE_EXTENSIONS
+    if helpers.ends filename, ext
+      answer = exports._compileFile filename, true
+      return sourceMaps[filename] = answer.sourceMap
+  return null
 
 # Based on [michaelficarra/CoffeeScriptRedux](http://goo.gl/ZTx1p)
 # NodeJS / V8 have no support for transforming positions in stack traces using