Fix detection of multiple pixiv links

If multiple pixiv links were posted in the same message, only the last
one was detected since the regex was too greedy.
Refactor plugin system a bit again

* The plugin system now support hot-reloading for changed plugins
* save event got removed, instead there are now enable, disable, join
and part events, which are much better suited for a general use-case
* intervals got removed too, they should be implemented by the plugin
itself and cleaned up with the disable hook
Fix relative path
Replace all relative paths by absolute ones
Remove second part handler

The part event got handled twice and in the removed case even
incorrectly; i.e. it got raised if any other user /parted too.
Fix output of nyaa file sizes

The default setting for the filesize module includes conversion to bit
units.

Switched to humanize for more possiblities and saner default setting.
Disallow emoting oneself