Add ability to encode 64 the flag images on mod_languages

Scope:
Add the ability to encode 64 the flag images on mod_languages. 

Why: Reduce the http requests per page...

How to test: Install a multilingual Joomla! with some dummy content. Enable the mod_languages module (Language Switcher). Check the options, play with encode. Everything should still work. Report bugs

I think you should be able to use the `$path_rel` argument to only return the file path. Like this:
`JHtml::_('image', 'mod_languages/' . $language->image . '.gif', null, true, true);`
Then you can skip the preg_match stuff.