Custom Shell Options

It should be possible to run a custom shell command on start of Guake. I use tmux pretty liberally in my setup and tmuxinator to manage a profile for my Guake shell.

I have found that I can set a custom shell using the following command:

```
gconftool --type string --set /apps/guake/general/default_shell \
/home/naftuli/.local/bin/tmux-thndr.sh
```

My custom shell looks like this:

```
#!/bin/bash

set -e

export TERM=screen-256color

source $HOME/.bashrc

mux start thndr
```

The last line is the only real important one, which starts a tmux session called `thndr`.

Please make it easier to set the command like this so as to have Guake use a tmux session.