Wrap whole runtime sections in scl enable

[irushchyshyn]

Fixes #27.

It should be possible to always wrap just entire specfile section instead of separate commands in scl enable (https://bugzilla.redhat.com/show_bug.cgi?id=1187230). Meaning, that language specific transformer plugins are no longer needed. The PR also includes adding more docstrings and some refactoring.

• [x] Wrap whole runtime sections (%prep, %build, %install, %check, %clean) in scl enable statement
• [x] Add integration tests for cli command
• [x] Move metapackage specfile creation out of Convertor, as it does not convert anything
• [x] Reorganize bin.py to show metapackage related options separately
• [x] Run tests with pytest-runner and check code style