Various improvements of Diffs

1. Now diffs don't show verbose lines (commands, logs etc) by default.
2. You can enable verbose diffs in settings.
3. and - symbols now are CSS pseudo-elements and are not copied if you want to copy part of code.
4. Now diffs use the same colors used in GitHub.
5. Little improvements on the way diffs are parsed.
6. When you parse a diff of multiple files, them are nicely separated.
7. Now diffs are formatted inside a table (this will help us when we'll want to add line-numbers and other features).

This fixes #403 #402