Filter assignee list and/or pass pass username to "assignee add"

[NathanKleekamp]

Issue Description

Type: feature request

Describe what happened (or what feature you want)

First off, I absolutely adore this plugin. It's first rate!

I'd love the ability to pass a username to Octo assignee add instead of popping open a pick list that filters while typing. Something like:

:Octo assignee add JimmyCrackedCorn

The filter while typing functionality can be slow because it seems to search the entirety of Github's user base. Might be also be useful to limit the assignees to repo members (if a private repo).

[pwntester]

Thanks for the suggestion, I have added support for passing a login to the assignee and reviewer commands. Will add it as well for labels