Basic image retrieval, cmdargs does not work

[engina]

This patch:

1. Retrieves the image resources in the blog post to assets folder
2. Modifies the blog post so that it uses the downloaded resources

Bugs:
Command line arguments does not work.

[parkr]

The command-line args will work once 2.0 drops, but sadly it's been mucked up because Jekyll 1.x uses Commander which uses symbol keys, but Jekyll 2.x uses Mercenary which uses string keys so it's a whole thing.

This looks pretty good! It'd be baller if you could separate this stuff out into a separate method so we can unit test it, then... you guessed it: add a quick test for it! Thank you!

[engina]

I did my best, though review this patch with grain of salt as I do not know Ruby at all (Yes, I've lost my Ruby virginity for this patch).

Note: I forgot to mention that this patch also REPLACES any class in PRE elements to "prettyprint".

These settings worked the best for me but side affects must be considered.

[parkr]

Why do this?

[engina]

To pretty print code snippets (duh!).

This may probably better be handled in some other stage -- i.e. when generating the actual HTML pages. Feel free to remove it if there's a better way to handle syntax highlighting.