Add options.maxBytes, which allows us to avoid inlining images that are too large

I love this module, but a nice feature would be to limit inlining images that are too large.  The mod_pagespeed folks at Google, who supposedly know a lot about this, [recommend a maximum of 2048 bytes for HTML files and 3072 for CSS files](https://developers.google.com/speed/pagespeed/module/filter-image-optimize#ImageInlineMaxBytes).

So I modified the README to recommend setting ```maxBytes``` to 2048.  I figure it's safer to just assume the lowest good value everywhere, since it'd be tricky to figure out whether a file is HTML or CSS.