Basic support for I18n contexts as requested by #2063

I've tried creating the basics for this. It can only read the translations using contexts (not extract them yet) and it only has the ```__x``` function for now (as proposed by @garas in issue #2063).
Well I have (for now) only included ```__x```. If the other code is good I'll add the other ones in the way @garas proposed it.

```php
// Extra gettext functions with $context
function __x($context, $singular, $args = null);
function __xn($context, $singular, $plural, $count, $args = null);
function __dx($domain, $context, $msg, $args = null);
function __dxn($domain, $context, $singular, $plural, $count, $args = null);
function __dxc($domain, $context, $msg, $category, $args = null);
function __dxcn($domain, $context, $singular, $plural, $count, $category, $args = null);
function __xc($context, $msg, $category, $args = null);
```