update regex remap docs
Document the remap.config '.include' directive
doc: fix various sphinx syntax errors
TS-2426: add the xdebug plugin

The xdebug plugin scans X-Debug header values and emits the requested
debug information. The plugin is named after the X- prefix of the
headers that is scans and emits.

The values we scan for are names of debug headers. The first header
to be supported is X-Cache-Key, which report the cache lookup URL
used buy the request. the second header supported is the Via header,
which causes a verbose Via header to be emitted for the request.
TS-2426: document the xdebug plugin
TS-2426: add xdebug plugin Makefile.am