Add bash completions by Aneurin Price.

These can be manually installed as needed by end users. They
have been added to the repository so they can be kept up to date
with the latest code.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1588