Machine-specific themes in their separate file

I added a colorschemes.example file which sets some custom environment variable
(prefixed with `$DOTFILES_`) that tell tmux, vim and zsh what theme to choose.

The actual sourced file (sourced in zsh/zshrc, as you could have guessed) is
`colorschemes`, the one that ends in .example serves only as a "template".

When I pull the repository, I should just `cp colorschemes.example
colorschemes`.
Update zsh, vim and tmux to fetch themes from env variables

Closes #16.