Diff Editor Support

[svenefftinge]

An open handler for diff editors should be registered from within the monaco package.
For the uri, we should use something simple like diff:/caption and the options need to contain the two uris, as well as other options that the monaco diff editor exposes.

[akosyakov]

other options that the monaco diff editor exposes.

I am not sure about this, e.g. whether editors are read-only or not should be decided internally based on resources.

[svenefftinge]

I was thinking of options like renderSideBySide as declared here https://github.com/Microsoft/monaco-editor/blob/4930b6a/monaco.d.ts#L3100

[svenefftinge]

Fixed with #392