Add bash autocompletion

[hagenw]

I added a bash autocompletion definition file to allow for

$ is a[TAB]
a          an         available

$ is not an e[TAB]
empty       equal       executable  existing

I also added a short description for global installation to the README. Do you think we should add a description for local installation as well (storing the file somewhere under $HOME and sourcing it in .bashrc)?

If you would like to include it with npm, we could have a look at npm shell completion.