core: Read site config also from files in site config.d.

Site config options are now derived from the files (in order, where options may be overridden by a later file):

priv/config => site/<sitename>/config => site/<sitename>/config.d/*

Where files from the config.d directory is read in alphabetical order, ignoring those beginning with . or ending with ~.
These files are usually backup or other irrelevant files.

Fixes #648.
core: add Jarimatti V to list of CONTRIBUTORS.