BASH completion script for accumulo commands

[ibilley7]

Created a BASH completion script in the assemble/bin folder called _accumulo_completions that allows the user to complete the accumulo, accumulo admin, and accumulo ec-admin commands to streamline usage and testing of Accumulo. Gives the user suggestions of what they can enter next given what's already entered on the command line. Created a README file in the assemble folder that describes the setup and usage of the script.

Closes #4808

[dlmarion]

I'm curious if there is a way to generate the list of commands during the build process so that we don't need to remember to update this file every release.

[ibilley7]

If this is a good enhancement to move forward with, I would need someone else to pick this ticket up as today is the last day of my internship.