Introduce pre-commit hooks

[falkoschindler]

To finally put an end to changing file endings back and forth, this PR introduces pre-commit hooks.

To install it on your system (once for all repositories you're working with):

pip install pre-commit

To install it in your local checkout (once for this repository):

pre-commit install

To check all files manually (every now and then):

pre-commit run --all-files

But once installed, pre-commit will automatically run before each commit or push, fixing things like training whitespace or wrong end-of-files. Changes will appear as unstaged changes.