refactor config

[thvitt]

Configuration is a mess. We have more configurable than need be, and build_dir etc. doesn’t work in all cases.

new config system should consist of:

• a file config.xml that contains just the paths for (a) the sources and (b) the build dir
• a stylesheet config.xsl that reads the file, generates all derived values and exports configuration for both xslt and xproc
• a parameter or option that points to the config file