Add a plugin system

[Martmists-GH]

Having a way to let users add plugins to add/extend snekchek would be a great improvement to this tool.

Plugin system requirements:

• Add linters/stylers/tools/etc.
• Add output methods
• Add command-line arguments

More to be added

[hargoniX]

maybe we could go for something like the flake8 plugin system:
http://flake8.pycqa.org/en/latest/plugin-development/registering-plugins.html

[hargoniX]

But rethinking our high customizable via one config file way, what if we would add a [plugins] to snekrc where the user can define from where to load all the plugins he wants to load.

[Martmists-GH]

we should use the flake8 approach instead