PicGrid dev TODOs

Various tasks

[ ] Make sure Docker version actually works
- Can PicGrid a directory
- Can webserve that griddified directory
- Make sure Docker container has imagemagick in it
[ ] How come sometimes pictures turn into a diagonally-subdivided gray rectangle?
- e.g. in http://picture-files.nuke24.net/uri-res/raw/urn:bitprint:IQXSR7HBOJNPWVAOXUCHZVKAKXPJNO4O.YZPCHLTNS4I3A4SAHJZC7ALQZH5EMD6B5X6YPUQ/Game21.html
[ ] New layouter method (see below)
[ ] Compose should allow filesystem objects to be passed on the command line and store them itself (should be able to steal directory serialization code from ccouch3)
[ ] Include a utility for recursively scanning emitted HTML files for URNs and writing them all to STDOUT.

[X] Layouter that runs both borce and rowly (and any new ones you come up with) and picks the most fit layout from all the results.
[X] Allow selection of layout from command-line
[X] Break program into smaller parts that can be run separately. (compose can accept directories or compound image files and can emit compound images, raster images, or HTML)
[X] Pretty picture for the README (2018-08-27)
- Maybe this one: http://picture-files.nuke24.net/uri-res/raw/urn:bitprint:32B7UZ4SORCYNVM7ZRPG5UBJA3DDR4TB.2EALXNOSI7JNBPIIM7TN6672WVGVGXAXM25OHNA/Mushroom.html