Allow specific exports to be overriden if wanted

For example I want to use globs (minmach) instead of dummy extension matching
```js
var stringify = require('stringify');
var minimatch = require("minimatch")

stringify.hasStringifiableExtension = function hasStringifiableExtension(filename, extensions) {
	var match = extensions && !extensions.every(function (ext) {
		return !minimatch(filename, ext);
	});
	return match;
};
```
Then I could use it like this, to exclude all the html starting with `_`:
```js
stringify(['**/[!_]*.html'])
```