Support installing optional sets of additional dotfiles

[jwodder]

Adjust the dotfile configuration so that the following sets of files are not installed unless a set-specific option is passed to dotdrop.sh or its replacement:

• Rust config files & cargo extensions
  • Also support templating ~/.profile based on whether this set is being installed
• Mutt config files
• APT-wrapping programs

Also support passing the options from setup.sh to dotdrop.sh.

Partially blocked by #9.