Allow customizing handled extensions

[shawninder]

Currently just supports .css out of the box. This should be configurable in case you want things like .css4 or .css.template if you have a templating postcss plugin or things like that

[shawninder]

Actually this is already available via the include plugin (pass a function and it will treat as CSS any file for which the function returns true. What is left is to support CLI usage, so if provided include is a string, require it and obtain the function...