Add multicursor autocompletions

[redsti-github]

Implements autocomplete while using multiple cursors. (loosely based on #3442)

Only autocompletes cursors which end in the same word as the active cursor.

For example:

foo bar
f|     -> foo|
f|     -> foo|
b|     -> b|
f|b    -> f|b

[JoeKar]

But we already have #3442 and this use case is discussed within it?!
I don't see any reason to duplicate the PR.