Bash Completion

- Improved bash completion when TASKRC is exported.
- Added a unit test.

Description of previous behavior:

  $ export TASKRC=/tmp/.taskrc
  $ task ad<TAB><TAB>

lead to the following output:

  $ task adTASKRC override: /tmp/.taskrc
  dTASKRC override: /tmp/.taskrc
  TASKRC override: /tmp/.taskrc

Signed-off-by: Paul Beckingham <paul@beckingham.net>