add boxen autocomplete

@dgoodlad

tab completion for boxen, knows all commands + servies + commands that accept directories

```
boxen -<TAB>
--debug             --future-parser     --no-color          --profile           --stealth
--disable-service   --help              --no-fde            --projects          --token
--disable-services  --homedir           --no-issue          --report            --user
--enable-service    --list-services     --no-pull           --restart-service   -?
--enable-services   --logfile           --noop              --restart-services  -h
--env               --login             --pretend           --srcdir

boxen --restart-service<TAB>
--restart-service   --restart-services

boxen --restart-service <TAB>
dnsmasq          memcached        nginx            solr
indexer          mysql            redis
```