Add agnoster theme from oh-my-zsh
Update powerline characters

* from https://github.com/robbyrussell/oh-my-zsh/pull/1556
* see https://github.com/keithw/mosh/issues/351 for details
* see a307ecd91e2e45143756cb7917f756cbc0f66806
zshrc: use custom agnoster theme
Merge branch 'features/agnoster'

fixes old vim-powerline character problem