Allow users to drop multiple branches in a single command. Instead of:

    $ git branch-list 1
    $ git branch-list 3
    $ git branch-list 4

you can now do:

    $ git branch-list 1 3 4

This makes it easier to drop a bunch of branches that you no longer need
saved.

drop_branch():
* No longer uses `branch_or_current_branch()`. Kept that function around
  because I didn't feel like touching the `save_branch()` code, and
  extracted the Git command to get the current branch to a new
  `current_branch()` function.
* Used one loop to get branch names from IDs if necessary and feed these
  into a Bash array. A second loop just after deletes all branches in
  the branch array using the `sed` method from before. Didn't feel like
  changing the `sed` command so did it this way. That said, we're
  probably going to have to come back to it sooner or later because it's
  not going to be able to handle branch names containing slashes.

main `case`:
Change `drop_branch` call to pass it all command line arguments, except
the 0th argument, which is the "drop" command (this is why we `shift`
before calling the function).

101-drop-accepts-multiple-branch-arguments.t:
Update test expectation which had an extra newline at the start of the
string which isn't there in the real `git branch-list` output.

In order to be able to easily reuse the `$BIN` variable we created in
the test for the `clear` sub-command, move it to a Perl module that can
be included in other tests.

Add the `t/` directory to the include path when running `prove` to
ensure that `bin.pm` can be found and included.

Thanks to these resources for explaining Perl modules:
https://perlmaven.com/how-to-create-a-perl-module-for-code-reuse
https://stackoverflow.com/questions/23899121/perl-declare-and-export-variables-from-a-module/23900384#23900384
https://stackoverflow.com/questions/17931981/what-is-isa-in-perl/17932340#17932340

A new sub-command that clears the entire branch list. This makes it easy
to remove everything when all the branches in the list are stale (e.g.
everything's been merged already).

Copied and modified from:

https://github.com/teddywing/git-hook-pre-commit-python-javascript-syntax-linter/blob/efa1909/t/001-setup.t
https://github.com/teddywing/git-hook-pre-commit-python-javascript-syntax-linter/blob/f6bb0d3/t/999-teardown.t