Support zsh “modes” (different keymaps like when using vicmd/viins pair)

Fixes #508
Some fixes for powerline.zsh:

- Replace main and other linked modes with modes they link to
- Add proper keymap initialization

Ref #508
Omit any non-vi* mode by default
Add tests
Modify appropriate theme
Merge pull request #709 from ZyX-I/zsh-vi-mode

Add support for modes to shell bindings