Adds support for external extensions for guard-templates

Hi,

Great work with guard templates :). I think it is important to allow users to add support for precompiled extension without modifying the gem and the Compilers class though.

This pull request provides an example doing so. I have developed https://github.com/sdrdis/guard-templates-jshaml which precompiles haml to js using this example. What do you think ?

Thanks,