WIP: Added option to save files in archive

[MrCyjaneK]

This should close #485, which I've opened a couple of months ago.

Usage is simple, just add:

archive:
 - test.txt
 - file2.txt

to your .abstruse.yml and files will be saved in ~/data/archive/{job id}.

Currently there is no way to serve the files with the api (code is written, it just doesn't work, probably an issue with chi?).

TODO:

• [x] Save files on server
• [ ] Provide a simple ui for browsing the archived files
• [x] Redirect from /archive/{repo name}/{branch/pull request id} to /archive/{job id}