Support for GitGutter

It uses the dict's diff.deleted, diff.changed, diff.inserted (https://github.com/jisaacks/GitGutter#readme)

I think adding the entries below in each is all that is needed. Using Base 08, Base 09 and Base 0B

```xml
<dict>
  <key>name</key>
  <string>diff.deleted</string>
  <key>scope</key>
  <string>markup.deleted</string>
  <key>settings</key>
  <dict>
    <key>foreground</key>
    <string>#AC4142</string>
  </dict>
</dict>
<dict>
  <key>name</key>
  <string>diff.inserted</string>
  <key>scope</key>
  <string>markup.inserted</string>
  <key>settings</key>
  <dict>
    <key>foreground</key>
    <string>#90A959</string>
  </dict>
</dict>
<dict>
  <key>name</key>
  <string>diff.changed</string>
  <key>scope</key>
  <string>markup.changed</string>
  <key>settings</key>
  <dict>
    <key>foreground</key>
    <string>#D28445</string>
  </dict>
</dict>
```