Add basic tmux configuration

Use tmux. 'tmux.conf' configuration file is added.
Summary of my new tmux configuration:

- key prefix is <C-a>
- screen-compatible and vim-like key bindings
- prettier and simple status bar and pane bordering

To make vim and the shell work better, default TERM is changed to
'xterm-256color' and vim's BCE option is configured.
Zsh aliases for tmux (tmuxa, tmuxl)
Tmux: No automatic renaming of windows and sessions
Merge branch 'tmux'
Add '~/.local/bin' into PATH before prezto startup

Add '~/.local/bin' into $path by putting it in .zshenv, so that
prezto can correctly initialize fasd. (fasd_cd was not working)