Allow haxeui-core to pre-process css styles

[ianharrigan]

It would be nice to have the ability for haxeui core to also be able to process .less and .sass files as part of the module processing. This would mean that .less or .sass files could be used in the <themes> section and auto compiled to .css (assuming lessc or sass was available - if its not available it should fail gracefully and simply ignore the entry)

Assuming this is implemented and working the default theme should also be rewritten to use .less/.sass files, however, it should also ship with the generated .css files (for users who dont have lessc or sass installed).

Apart from variables and imports this could have quite a large impact of allowing a new theme (that would extend the default theme) to simply override a, say vars.less file which contain a list of colours and metrics (border-size, border-radius, etc) meaning a visually different theme could be created with very little effort by simply (re)defining vars in the default theme.

Ive already checked with the lessc and sass compiler and custom css attributes are totally fine (which is 100% required).

Finally, it would be important to not actually link this to either less or sass but have a generic way to run "style plugins" defined in module.xml to be run at compile time. This would allow other pre-processors not defined in haxeui-core to be run