Add filters to modify the cached URLs

[carlalexander]

This is PR is part of a fix for #44. It adds two filters to the amber link caching process. One during the enqueue process and another during the lookup. This lets us edit links before Amber tries to cache them. We then modify those links again when Amber looks them up.

Here's an example of the use of these filters to fix the issue in #44:

function remove_twitter_url_query_string($url) {
	return preg_replace('/(https?:\/\/twitter\.com[^?]*).*/i', '$1', $url);
}
add_filter('amber_cache_link', 'remove_twitter_url_query_string');
add_filter('amber_lookup_url', 'remove_twitter_url_query_string');

[carlalexander]

Just a note on the Travis errors. It seems like the Travis config doesn't work for PHP 5.3 or HHVM. HHVM could probably be dropped since WordPress doesn't support it anymore either.

[jlicht]

Suggested change

	$item = apply_filters('amber_cache_link', item);
	$item = apply_filters('amber_cache_link', $item);

[jlicht]

Suggested change

	if (empty($link)) {
	if (empty($item)) {

[jlicht]

Thanks for this pull request! Can you give us permission to make modifications, as per https://help.github.com/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork/