Feature: Automatic installer for `zsh` and support for missing `.bashrc`

[mfhepp]

It may be useful to add automatic installation for ZSH to the installer.

Blockers:

1. It is non-trivial to decide for the users as for which shell or shells they want to install it to.
2. The absence of a .zshrc or .bashrc is no reliable indicator of the proper shell, as it may have to be created.
3. The usage of the proper one of the several files is a bit complex and may not be platform-independent (e.g. .bashrc vs .bash_profile on OSX).