Allow users to set `bash_completion_path`

[ragu-manjegowda]

Let users set the completion file path, in current format it needs us to manually move the file to specific location.

I have installed bazel with linuxbrew and it sits somewhere in /home/linuxbrew/.linuxbrew.

Instead I can keep a copy of bazel-completion.bash somewhere like ~/.config/bash and just set bash_completion_path in vimrc