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.

lib/coffee-script/coffee-script.js

@@ -1,8 +1,7 @@
 // Generated by CoffeeScript 1.10.0
 (function() {
   var Lexer, SourceMap, base64encode, compile, ext, fn1, formatSourcePosition, fs, getSourceMap, helpers, i, len, lexer, parser, path, ref, sourceMaps, vm, withPrettyErrors,
-    hasProp = {}.hasOwnProperty,
-    indexOf = [].indexOf || function(item) { for (var i = 0, l = this.length; i < l; i++) { if (i in this && this[i] === item) return i; } return -1; };
+    hasProp = {}.hasOwnProperty;
 
   fs = require('fs');
 
@@ -369,15 +368,19 @@
   sourceMaps = {};
 
   getSourceMap = function(filename) {
-    var answer, ref1;
+    var answer, j, len1, ref1;
     if (sourceMaps[filename]) {
       return sourceMaps[filename];
     }
-    if (ref1 = path != null ? path.extname(filename) : void 0, indexOf.call(exports.FILE_EXTENSIONS, ref1) < 0) {
-      return;
+    ref1 = exports.FILE_EXTENSIONS;
+    for (j = 0, len1 = ref1.length; j < len1; j++) {
+      ext = ref1[j];
+      if (helpers.ends(filename, ext)) {
+        answer = exports._compileFile(filename, true);
+        return sourceMaps[filename] = answer.sourceMap;
+      }
     }
-    answer = exports._compileFile(filename, true);
-    return sourceMaps[filename] = answer.sourceMap;
+    return null;
   };
 
   Error.prepareStackTrace = function(err, stack) {

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