purple-mattermost libpurple plugin

Author:

Eion Robb <eion@robbmob.com>

Contributors:

Edvard Rejthar <github@edvard.cz>
it33 <it33@users.noreply.github.com>
Jarek Polok <jaroslaw.polok@gmail.com>
Patrick Laxton <patrick.laxton@protonmail.com>
Peter Chubb <Peter.Chubb@data61.csiro.au>
