mostly names and data structures.

These are emphasizing markups per the "Markdown standard".

A line containing two '*' or '+' or '-' are not valid horizontal rules.
See: https://daringfireball.net/projects/markdown/syntax#hr

these settings are used to change the order of the posts on the listings.
by default, users are supposed to list their posts from older to newer in
the blogcfile, and blogc-make will list them on descending order, both for
html listings and atom listings.

when looping through the tree, the algorithm would stop, if found a '\0'
in the key of the tree node. there should be no "child" field after a '\0',
but "next" fields may exist.

now any directories listed in the [copy] section of blogcfile will have
its files copied recursively to the output dir. this is useful for big
chunks of generated files, e.g. doxygen documentation.

now it is only possible to override output directory using environment
variable. this commit also reduces the number trie lookups.