Install all "official" plugin

[ovv]

@mrasband proposed to have an easy way to install a bundle of plugin. We don't have much at the moment but a clean way to install/update all "official" plugin at once is a nice to have.

Something like pip install sirbot[all] or pip install sirbot-all

Linked to #19

[mattrasband]

I think we have a few we may want, fortunately this is easy with the extra_requires feature of setup.py! They can be combined with commas too: pip install sirbot[slack,logging]:

The few in mind that could make sense

1. slack -> include the slack connector
2. irc -> include the hypothetical irc connector
3. pyslackers -> our preconfigured and curated bot functionality