Add support for filtering processes by substring.

[rephorm]

Press / to input a filter string. The proc list will be filtered to
procs that contain the filter as a substring of either the command or
the string representation of the PID.

[cjbassi]

Nice, this is something I've wanted for a while.

So a few things I think we should do:

• rerender the process list after every keypress, since it currently only rerenders on the next tick
• make <Esc> cancel the filtering
• maybe we shouldn't consume the <Tab> keypress, so users can still toggle process grouping when filtering
• add a cursor to the filter prompt
• if no processes are displayed, the scroll indicator should display '0 - 0 of 0' (sometimes it gets a little buggy)

[rephorm]

Great suggestions! Implemented.

I reworked the event handling to not swallow events other than those specifically handled. So, <C-u>, <C-d>, etc also work now while editing.

<Escape> doesn't seem to be sent immediately (presumably waiting for terminal escape codes?), so it feels laggy compared to <C-c>.

[cjbassi]

Wow, really nice. Quite a few changes so it'll take me a little while to go through.

Btw, <Escape> is working fine for me with the filtering.

[rephorm]

Take your time! I'll stop updating this branch. :)
I added cursor movement (via <Left>, <Right>) in the scroll branch if you're interested.

[rephorm]

I should have clarified earlier that the 3rd commit undoes many of the changes in the first commit. It will probably be easier to review changes from all commits instead of individually reviewing.
If you would like me to squash the related commits in a separate branch to simplify things, let me know.

[rephorm]

Ping

[cjbassi]

Sorry for the silence. This is looking really nice! Seems like all the previous suggestions were implemented. I looked over everything and it LGTM so feel free to add in anything else you've done. One thing is if you could sign #151 and license your additions under MIT that would be great since I'm in the process of relicensing.