[FEATURE] Allow to install plugins via pip

[MikkCZ]

Beside git, pretalx plugins can be also installed as pip packages. This ansible role allows to install pretalx itself as a pip package, but plugins are only installed from git. Btw. pip should be able to install packages from a git repository as well, so there is a potential to just use pip for installation regardless of the source, whether it should be a release from PyPI, or bleeding edge from a potentially custom git repository.

[MikkCZ]

https://pip.pypa.io/en/stable/topics/vcs-support/

[MikkCZ]

I am not 100% sure to make the change in a backward compatible manner, but I am in general interested in such feature to install plugins as packages instead of cloning remote repositories. Will see how fast I can get to this through my personal backlog.