Compare to index

1. Create a new editor option that controls the visibility of the diff margin. Future versions of the extension could implement a UI for controlling this value, and it will automatically be functional.

2. Implement three new commands for controlling the base version used in comparison operations. The commands are defined and implemented, but not exposed in the UI.
    * **Compare to Index**: The working copy is compared to the index (where files get staged).
    * **Compare to Head**: The working copy is compared to the latest commit.
    * **Toggle Compare to Index**: A single command which toggles between **Compare to Index** and **Compare to Head**.