Enable bash completion

[jmccann]

Enable Drone CLI to generate bash completion output. Combined with user copying bash completion script following https://github.com/urfave/cli#distribution this enabled autocomplete with Drone CLI!

Example

$ ./drone 
build     exec      help      registry  secret    
deploy    h         info      repo      user
$ ./drone build 
approve  h        info     list     queue    stop     
decline  help     last     logs     start   

And yes, it does autocomplete drone b -> drone build when tabbed and such. Bit harder to show example of that 😛

Fixes #40

https://github.com/urfave/cli#bash-completion