Invoke vim function if g:ctrlp_user_command starts with '*'.

Use user-defined vimscript function if `g:ctrlp_user_command` starts with '*'.

In windows, globapath() is very slow. So I want to use another function such as `if_python` interface.

[User function sample](https://gist.github.com/sgur/5806782)
This sample use if_python interface to glob files (and considering depth and ignoring patterns).

The function is almost x2 faster than `globpath()` in windows.