Reset the terminal when Ctrl-C is pressed

[lambda]

If a user hits Ctrl-C to exit out of a search in the middle of printing
a line, we don't want to leave the terminal colors screwed up for them.
Catch Ctrl-C using the ctrlc crate, obtain a stdout lock to ensure that
other threads don't continue writing after we do so, reset the terminal,
and exit the program.

Closes #119

[lambda]

This PR is a bit of a strawman; I figured I'd try the simplest possible thing that seemed to work. I first tried to see whether I should thread this into ColoredTerminal somehow, but that seemed complicated, and this approach seemed simpler.

I haven't tested this at all on Windows, so don't know if this will work properly on there.

On my system, for some reason, this suppresses the shell prompt printed after exiting. I haven't quite figure out why that is. But it does have the effect of resetting the colors appropriately, so if I just hit "return", I do get the prompt properly. Writing out extra newlines and flushing doesn't seem to help this.

I had expected that this would have the effect of losing the information about the process being killed by a signal, just returning 1 instead, since I didn't clear the signal handler and re-raise the signal, but echo $? gives me 130 as you'd expect from an interrupted process. So I guess that's good, if unexpected.

[BurntSushi]

@lambda Nice! Thanks so much for doing this. I'll pull this down and try to play with it soon before merging. (It may not happen until this weekend, sorry!)

[BurntSushi]

OK, I tried this out and it does indeed fix the issue for me. Interestingly, I'm not experiencing the shell prompt suppression. I get my shell prompt back, but I have no idea what the forces are at play here. (FWIW, I'm running bash in konsole as my terminal emulator on Linux.)

I merged this in 811fcc1 (after a rebase).

Thanks so much!