Ignoring files

[tony]

I want to build file-ignoring into pyvim

1. I need a solution where I add a basic function in my ~/.pyvimrc that takes a filename and accepts/rejects it
2. It should work be able to pass in to https://github.com/jonathanslenders/pyvim/blob/f428949924eabe61fca18a54621c475fe33f05d2/pyvim/commands/completer.py#L22 file_filter.
3. We should allow configuration to potentially add multiple ignore functions. For instance, I may want to ignore .pyc and make a function for that, but I may also want to in the future be able to ignore .gitignore globs.

I'm going to suggest a PR first and get feedback on it, best.