Use emacs diff colors

[Azkae]

Improve syntax highlighting by sending the emacs diff colors to delta.
This way, the colors should better match the background.

I have also added magit-delta-max-size to avoid running delta on large files since it's easier to do now (I can move it to later PR if you prefer).

This should also fix #28.

A few notes:

• magit-delta-mode is now a global mode.

• Since we are using diff faces, we need to have the correct faces already loaded when enabling magit-delta-mode. For example, if we enable magit-delta-mode before setting up a theme, we would use the wrong colors.

  Using (add-hook 'magit-mode-hook (lambda () (magit-delta-mode +1))) works since by the time we turn on magit mode, the faces should be correct.

Before:

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After: