Blackhole v0.3

Please, do not merge until I release 0.3. I'll notify here.

This is meta pull-request with Blackhole's release candidate version.

Features briefly:

 * String formatter now can have optional placeholders.
 * Got rid of that annoying empty braces `[]` in **others** `%(...)s` placeholder.
 * External rotation-aware file sink.
 * Improved performance.

For more detail please visit: https://github.com/3Hren/blackhole/blob/master/debian/changelog
I'd say the readability of that config line hasn't changed.