Insert a small sleep when polling stdin

Resolves #48

Here is how to reproduce this issue

$ tl --version
tl, version 1.4.0
$ echo '{"foo": "bar"}' > /tmp/log.json
$ cat  /tmp/log.json | tl

This is what happens in main now. 100% CPU usage when piping

I added a small sleep when there is nothing to read. I tested 1 ms and 5 ms.

1 ms used about 2% cpu.

5 ms used about 0.7% cpu.

I would guess 5 ms will not be very noticable, but if it is we can go down to 1.

Testing

I tried piping in a 40MB file and did not notice any difference in performance with or without the sleep.