split shtab/__init__.py?

[ThomasWaldmann]

Maybe it would be a good idea to split that:

• separate python code files, 1 per shell (currently: bash, zsh, tcsh)
• common code used by multiple shells either stays in __init__.py or is moved to common.py

First merge PRs that add support for a new shell to avoid merge conflicts.

[casperdcl]

currently part of #196 :)