Provide ability to change less compiler, specifically to less.php

The leafo/lessphp branch is no longer actively maintaned. As such, the latest features of the less compiler are missing (eg. the extend keyword). This becomes an issue when working with frameworks that take advantage of these new language constructs. Bootstrap > 3.0.0 will not compile with lessphp (https://github.com/leafo/lessphp/issues/535). This issue has been hanging around for a while now, but no progress has been made.

This Pull Request addresses this issue by allowing the ability to use the 'lessc.inc.php' that is now part of the less.php project to provide compatability with lessphp projects. It allows the choice of compiler through a constant that can be added to the wp-config.php file, WP_LESS_COMPILER.

The options for this variable are "less.php", which will include the above mentioned include. Since this is the primary (only?) alternative right now, it is included as a submodule in the same 'vendor' directory as lessphp.

The issues with using the different compiler are 1) performance and 2) no support for php defined functions added via registerFunction.

For backwards compatibility I think this should be an opt-in on behalf of the developer at this point because if they are currently using the registerFunction feature of less.php, it will break their theme.