bash: Rework configuration and path export

Split `bash_exports` into two files:

* `bash_config` is where custom Bash configuration is set.

* `bash_paths` is where path modificiations are made.

The private/custom settings in `bash_profile.local` are sourced as late
as possible (so that those settings can override any set in the
dotfiles), but not before the `bash_paths` file. This ensures that `brew
--prefix` is known to `bash_paths` if Homebrew was installed in a custom
location.
bash: Minor improvement to comment consistency
bash: Break `bash_functions` into separate files
bash: Make `cleanup` a generic function

Replace the `cleanup` alias with a more flexible function that deletes
all `.DS_Store` files by default or, if a pattern is specified, all
files that match it.
vim: Load `.vimrc.local` last

The settings from `.vimrc.local` should be loaded last so that they can
overwrite all other pre-existing settings in `.vimrc`.