An initial pass at allowing drops to be downloaded

A super rough pass at adding download support for drops. So far constraints are that URLs are selected based on a `cl.ly` domain (based on our convos), and the last argument is optional to specify a target directory for the download.

:triumph::sweat_drops::clap:

I didn't get to look and see if you provide the account holders custom domain, if so, could add that as part of the `cloudapp_url?` filtering. :eyes:

Also, eventually perhaps an additional query to the API to provide drop data based on its slug. Currently only by ID a drop is served back. :question: 

Usuage:
`cloudapp http://cl.ly/3W0R2X0E1t1U http://cl.ly/472M040N3Z44 ~/Desktop`

Supports existing piping, and output from my testing...
```
Downloading http://cl.ly/3W0R2X0E1t1U... /Users/dan/Desktop/helmet-what.gif
Downloading http://cl.ly/472M040N3Z44... /Users/dan/Desktop/blinking-homer.gif
```