Several (minor) changes.

Because I forgot to commit between each change.

- run_list now returns a string list, not a string list option
- Sources.add_subcommand now expects a Sources.t Lazy.t instead of a Sources.t
  The startup time of the examples is *much* better.
- I also added some new subsources in "examples/with_subcommands" these are
  specific to my system, but I'm to lazy to create my own dmenu on the side.
  I don't think that will pause a problem to anyone, but if it does let me know
  and I'll revert.