Add a command for managing the cache

[shawnmjones]

A command that allows the user to manage the cache would be very helpful after we have implemented #65.

I'm envisioning something like the following:

This command would list all URIs in the cache:

# hc-cache list-uris -o all-uris.txt

This command would purge all cache tables:

# hc-cache purge-all

This command would only purge the memento URI-Ms in the list:

# hc-cache purge -i memento-urims.txt

This command would only purge the cached content of memento URI-Ms, but leave the derived data:

# hc-cache purge -i memento-urims.txt --only-content

This command would preload the cache with a list of URIs:

# hc-cache preload -i uris.txt

This would export the cache into some (to be determined) file format:

# hc-cache export -o exported-cache-data.dat

Likewise, we can load the cache using some (to be determined) file format:

# hc-cache import -i some-elses-cache-data.dat

As time goes on, I'm sure I can think of other things.