Overhaul xkcd-fetch

[lramati]

scared and I are going to completely rewrite xkcd-fetch, both the interface and internally to address a number of issues revolving around it. I'm just making this issue to clean things up.

This will directly address issues #20 and #13 and will hopefully address #9 in the process.
#20 addresses some ambiguity in the help message,
#13 addresses something that can be cleaned up in the storage format for convenience,

and #9 deals with the cache possibly corrupting itself (we have yet to reproduce this error though)

[jgonggrijp]

I replaced this issue with a milestone.