Add download support

This adds a new "download" command that will download a named asset from
a release to the current directory.  Both public and private repos are
supported.  Info command now also supports private repos to simplifly
listing assets to download over the command-line.
Merge pull request #5 'Add download support'
download: formatting + error-check file creation
fix bug in the 'info' cmd, make it more robust

One of the arguments wasn't being sourced correctly. I'd like to find a
better solution to the current argument duplicating because it's not DRY and
leads to these kinds of copy-paste bugs.

The command should detect failure a bit better now too.