[wip] implement dirlisting (but yet does not actually return a list)
build directory listing
bundle picotemplate as a submodule (not subtree since it is not required in dist)
add helper script to run all generators
do not use the non-standard d_namlen
refactor
support returning the directory list w. configuration directive `file.dirlisting` (equiv. to `Options Indexes` of Apache, merge branch 'kazuho/dirlisting')