File name sanitization as pre-commit hook

[Robbert]

Stuff like:

• [x] only ASCII file names, no control characters et cetera.
• [x] not overly long.
• [x] prevent system file names.
• [x] ignore file names in node_modules

I've worked on a nih-filename module that performs similar checks, maybe use that one.

[Robbert]

In the meantime we can use the npm module sanitize-filename and check wether sanitize(filename) !== filename.

[Robbert]

Added a Gulp task that uses the sanitize function, and added check for the filename length.