Added syntax sensitivity to DashDoc

- DashDoc can now instruct Dash to consult a a docset specific to the syntax of the current view (default: off; a mapping between syntax and docset can be provided)
- Added key binding Ctrl-Alt-H to enable syntax sensitivity in a one-off fashion
- Added DashDoc settings file
- Added DashDoc entry under ST2's preferences menu
- Added DashDoc commands to ST2' command palette
- Adapted the README