Precedence for registered Less function handlers over built-in handlers

<a href="https://github.com/s7eph4n"><img src="https://avatars.githubusercontent.com/u/2829850?v=2" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [s7eph4n](https://github.com/s7eph4n)**
_Sunday Aug 17, 2014 at 19:02 GMT_
_Originally opened as https://github.com/oyejorge/less.php/pull/161_

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All this patch does is to first check registered functions before trying for a built-in function. This will allow to re-define builtin Less functions with own handlers.

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_**[s7eph4n](https://github.com/s7eph4n)** included the following code: https://github.com/oyejorge/less.php/pull/161/commits_