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

Default value for the encode param needs to be 0 (or don't specify) so it's consistent with the default value in the XML and with existing behavior.
Thus please change to `if ($params->get('encode'))`