[Plugin] Use dict for settings

* Moved the settings in a dict rather than a lot global variables
* Added a default settings dict
[Plugin] Add live hightlight support for conflict groups

* Groups are "ours", "theirs" and the diff3 type "common ancestor"
[View] Add gutter theme icons
[Plugin] Highlight the conflict groups with gutter marks
[Doc] Updates the README to explain the configuration possibilities
[Doc] Add a message for the new version
[Plugin] Add life hightlight support for conflict groups

* Three different gutter marks for three different groups
  (ours, theirs, common ancestor)
* Adds configuration options in the options file
* Updates the README appropriatly