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`.