Add command dodo setup

[mnieber]

This command should make it easy to create a good Dodo Commands setup after the installation:

1. It will create a temporary prototypes directory for prototypical files and snippets. At the same time, it will create a temporary symlinks directory with symlinks to the current files, such that a diff between the two directories allows people to merge in the prototypical settings.
2. It should create an example snippet for .bashrc or .bash_profile in prototypes. This snippet loads the .dodo_autostart file. On MacOS, it also runs the eval "$(register-auto-complete dodo)" line. Finally, it has the line to extend the PATH to include the dodo executable.
3. It will check if meld is installed, and offer to install it.
4. It will add a global dodo config to prototypes that sets meld as the diff tool, and adds aliases.
5. It fires up meld to compare prototypes to symlinks.