[zshrc] Add RVM to PATH for scripting
[zshrc] add rvm plugin
[zshrc] export env vars before loading oh-my-zsh

- Putting the env exports before loading oh-my-zsh helps some plugins.
- Update the PATH to account for the default system path.
- Put PATH export in if statement to avoid duplicate paths in tmux.
[zshrc] load rvm
[zshrc] include vagrant oh-my-zsh plugin
[tmux.conf] remove binding to reload tmux.conf
[tmux.conf] allow tmux to use system clipboard
[vimrc] don't show line numbers