Add script to submit plugins to the addons site.

This mirrors the functionality in the build-plugins.xml ant project
in the main repo, but as it's not using ant is an awful lot nicer
and more sane.

We'll use this to submit plugins from teamcity builds.

Change-Id: I5d88c51bb90593d479bdff03d06c6f4dbba2d651
Reviewed-on: http://gerrit.dmdirc.com/3355
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>