Bug 14727: koha-indexer tab completion in bash

Bug 14727: koha-indexer tab completion in bash

This patch introduces tab-completion in bash to the koha-indexer
command. It completes using valid option switches and instance names.
It stops completing then -h or --help are issued, and once a command
switch is chosen (either --start, --stop or --restart) it goes ahead
with valid instances completion.

To test:
- Have a packages install (kohadevbox works)
- Run:
  $ . debian/koha-common.bash-completion
- Type
  koha-indexer <tab>
- Play with all the options
=> SUCCESS: They work as they should.
- Sign off :-D

Note: it fixes a small glitch on the plack completion.

Signed-off-by: Jesse Weaver <pianohacker@gmail.com>
Verified working, including filtering out already used options and
instances.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>