Re #3: Reworked shell identification

Hi, I don't make pull requests too often, hope this way is okay.
I reworked shell identification and tested it on both bash and zsh on Macbook and Mint Linux. Identification seems to work fine now. Please have a look if this also works for you.
The multi-shell-repo-prompt is not working for me, though (I'm normally a bash user), so I'm not sure what's up with that (the prompt in zsh on Macbook is literally "$(construct_prompt)$msrp_reset_color").