Install formatter with Behave (via extras_require)

[bittner]

If we want to make it easy to include this formatter with a Behave installation we should add it as an optional dependency in the Behave project, e.g.

# FILE: setup.py
...
    extras_require={
        'html': ["behave-html-formatter"],
...

Install instructions would then look like, e.g.

python3 -m pip install behave[html]

Formatter Auto-Detection

Also, we may want to consider implementing some kind of auto-detection for formatters like ours in Behave, to avoid cumbersome setup instructions and puzzled users.

[jenisys]

@bittner
Just provide a pull-request in the behave project for "setup.py" (with the changed snippet and the reasoning from here).