Optimize output panel by writing less often

[kaste]

Instead of writing sync on 'write', write on 'flush' and 'close'.

[kaste]

After #150 I was wondering if the output panel is a bottleneck here. With this small change, we can already make the tests faster without loosing much.

def flush(self):
	self.set_timeout_async(self._write)

would be a more aggressive solution. With that the test_user_args test from SublimeLinter runs in 0.5s (master: 6s, #150 1s).

But users prefer sync writing while tests are running even if the output runs through faster than one can read.

[randy3k]

This PR looks good and I agree that self.set_timeout_async(self._write) is a bit too aggressive. Let's merge this first and consider the improvement later.