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.

[redsti-github]

Yes, but #3442 still autocompletes on all cursors, even if they shouldn't be.
I created this PR because as far as i know, i can't add or suggest commits to someone else's PR. (Atleast not without opening a PR to the author's repo, or commenting a diff, both of which feel impractical to me.)