Prior to this change, completions only allowed one completion per command, this change allows as
many as required. The one downside to this change is the completion engine isn't smart enough to
determine which options are no longer legal after certain options have been applied.
Now when one completes an option with possible values, those values will be displayed. Imagine
a program with an `--editor` option, which accepts either `vi`, or `emacs`. The following would
be displayed for completions
```
$ prog --editor <tab><tab>
vi emacs
```
Closes#557
By using a build.rs "build script" one can now generate a bash completions script which allows tab
completions for the entire program, to include, subcommands, options, everything!
See the documentation for full examples and details.
Closes#376
