reworked config generators to call `context.resolveInFile(fname)` instead of joining path themselves
reworked ConfigWriter source resolution: now considers block.searchPath, root (which can be an array), and then file.searchPath
added warnMissing option, which issues warning if ConfigWriter can't resolve a source reference in a block.
added resolveSource() option, allowing applications to specify custom rules for resolving source urls.
pass options object to ConfigWriter