Sync with pty process using XOFF and XON

[Tyriar]

Fixes #425

This PR adds the following:

• Implements XOFF (ctrl+S) and XON (ctrl+Q)
• Separates writes from the processing of the writes by means of a write buffer (Terminal.writeBuffer) and setTimeout(...,0) to start the write async
• XOFF is sent to the pty process when a threshold is reached so that xterm.js can catch up
• Writes are batched in separate async calls in order to prevent the possibility of overloading the UI thread. Calling them all async is bad for performance as it means that the renderer would need to catch up before every write.
• Frames are now skipped in order to save CPU time for the processing of data, not useless rendering of output that will be replaces in ~1000/60ms

Notes:

• Since things are not synced with the backing process programs such as time are now accurate!
• ^C is now responsive
• There is a large diff due to an indentation change in write (now innerWrite), you only need to look at the beginning and end of the function though

[Tyriar]

Here are results from the perf test in #422 (comment) to show how this affects perf:

	Before perf work	CircularList	XOFF/XON	gnome-terminal
Print 1..500000	55s	8s	12s	3s
ls -lR ~ (524582 files)	68s	29s	22s	7s

Keep in mind while there is a regression, the UI is now completely responsive (at least on my laptop with these config values) and you can SIGINT the process at any time.

[Tyriar]

Also this is what an ls -lR /usr/lib timeline looks like which I've been using in other issues:

[Tyriar]

Ready for review