Add zsh support

[DE0CH]

Currently, the shell script only edits the ~/.bashrc file, but some people (like me) prefer to use zsh, which requires editing ~/.zshenv. This PR adds some code to detect the user's login shell and edit the corresponding source file.

Also I renamed BashFile to RcFile to make it more generalised. This will change the public interface because the BashFile env no longer works. Not sure if there needs to be backward compatibility because there seems to be no mention of using BashFile in the documentation.