Optionally display the commit buffer with magit

Introduces a new custom variable: `mo-git-blame-use-magit':

Controls whether or not magit will be used. Possible choices:

  `never'        -- do not use magit even if it is loaded
  `if-available' -- use magit if it has been loaded before
  `always'       -- automatically load magit and use it

Inspired from `mo-git-blame-use-ido'.
Merge pull request #12 from Niluge-KiWi/display-commit-buffer-with-magit

Optionally display the commit buffer with magit