Adding Tmux Plugin

This adds a plugin with several options for the program tmux, a terminal multiplexer.

The options available are:

1. Autostart: automatically start a tmux session upon logging in. Disabled by default.

2. Autoconnect: when running `tmux` automatically connect to the currently running tmux session if it exits, otherwise start a new session. Enabled by default.

3. Autoquit: close the terminal session when tmux exits. By default this is set to the value of Autostart.

4. FixTerm: When running tmux, the variable `$TERM` is supposed to be set to `screen` or one of its derivatives. Unfortunately, tmux has now way to say "if the host terminal supports 256 colors, make TERM `screen-256color`, else make TERM `screen`. Setting this option enables that behavior. Enabled by default.

If the user specifies any arguments for tmux then the command is just executed as the typed it without any modification.