Fix output filter

Fixes #6 (closed).

This produce following entries in .git/config

[diff "notebook"]
	command = git-nbdiffdriver diff --ignore-details --ignore-outputs --ignore-metadata
[merge "notebook"]
	driver = git-nbmergedriver merge %O %A %B %L %P
	name = jupyter notebook merge driver
[filter "whitespace"]
	clean = sed -e "s/\\s\\+$//"
[branch "fix_output_filter"]
	remote = origin
	merge = refs/heads/fix_output_filter

and install pre-commit hook that strips notebooks from all outputs and metadata.

I added pre-commit hook as a blob of text inside setup to make it available after cloning already initialised project. Other solution would be, I guess, to wget current pre-commit hook from repo.