Add button to stop/start recording

[mhils]

Turn flow recording on and off via a single button click.

[mhils]

This is actually more difficult than one may would expect:

1. If we just shut down the proxy in regular mode, all clients will just fail.
2. We could just turn on --ignore .*. In regular mode, unencrypted http:// requests would still go through mitmproxy then though (see http://docs.mitmproxy.org/en/latest/features/passthrough.html#explicithttp)
3. We could process stuff as normal, but just don't store flows on the Python side. This would still break certificate pinning.
4. We could do a combination of 2+3, but that doesn't make things less complex.