Fix environment variable leak for unused formatters

[krassowski]

Fixes #337

Previously the environment was polluted by rpy2 variables even when no R formatter (styler nor formatR) was installed. This was first happening on import of formatters.py and then when checking if these formatters are importable because each of these steps contained an rpy2 import.

With this change rpy2 is only imported when needed.

[krassowski]

All green!

[andrewfulton9]

Code looks good, and tests passed for me!