...when invoked with no subcommands.

Idea from joeyh.

Adds App::MarkFile::check_collisions().  Right now this just bails if
something would be weird.  An option to solve interactively is probably
a good idea, but for now this will at least prevent the obvious
foot-shooting case.

Of course it should probably also list the overlapping source paths...

...or where sources have overlapping names.  Which is kind of the same
thing, but kind of not.

- Moves more stuff into App::MarkFiles.
  - Renames each() to each_path() and simplifies the interface from
    handing a hashref to the callback to just giving it a path string.
  - Adds mark-remove(1) and renames App::MarkFiles::unmark() to remove()

This seems like it could get confusing as a basic command (I can see
someone mixing it up with mark-clear), and xargs is there if you need it.