Add support for running sub-processes under a PTY

[alecthomas]

This is really nice for running programs that output colour, such as coloured log lines, etc.

I defaulted it to off because some applications when run under a PTY will attempt to move the cursor around, clear lines, and so on, so it should be up to the user to decide whether to enable it.

[alecthomas]

Ping?

[mattn]

This break Windows behavior.

[alecthomas]

This should be fixed, though I have no means of testing it.

[alecthomas]

Ping.

[alecthomas]

BTW if you're generally against the idea for whatever reason, I'm happy to abandon the PR. It's a nice feature that I've been using personally, but it's your project after all :)