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

Closes #3442

[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.)

[Neko-Box-Coder]

Tested the changes, works well as expected. Nice one.

[Neko-Box-Coder]

Hey @redsti-github , I want to merge this to my own fork.

Could you tidy up the commits please?
i.e. "fix plugin support" and "remove trailing whitespace" should be in "Autocomplete support for multiple cursors" instead of having their own commits.

Thanks.