reformat & refactor, rename cmd.scm to docco.scm
add command-line submodule & remove easy-args dependency
minor css tweaks, increase code font size
error message argument formatting
add barebones markdown generator
downcase the output & language options

These are matched as lowercase symbols, so downcase them as soon as
they're collected.
use let-values instead of call-with-values

This makes the main HTML/ANSI generation loops slightly cleaner, IMHO.
unimportant formatting tweaks
refactor reader into various smaller parts

This large change does several (i.e. too many) things.

Firstly, it formalizes the previously-ad-hoc generator pattern used by
the reader to yield source sections.

Secondly, it factors a lot of inline comparisons and string munging out
into reasonably-named SRFI-13-alikes and a few other slightly more
general functions. This is at a performance expense, but readability won
this round, and it buys us the ability to do the third and final
thing...

Thirdly, it modifies the section reader to take a list of comment prefix
strings, rather than a single one. This allows us to handle multiple
line comment type for a given language, such as both ";;" and ";;;" as
is common in Lisp/Scheme.