Add next and previous commands for highlight references

Implement http://vfpt.uservoice.com/forums/247560-general/suggestions/6183603-tab-to-next-reference.

These are two native commands in VS: `Edit.NextHighlightedReference` and `Edit.PreviousHighlightedReference`.
The default key bindings in C# editor are Ctrl Shift + Arrow Down and Ctrl Shift + Arrow Up respectively