filter: introduce "filter type" prefix

This allows different filter implementations to be specified in the
configuration file.  Currently only "exec" is supported, but it may now
be specified either with or without the "exec:" prefix.

Signed-off-by: John Keeping <john@keeping.me.uk>

cgitrc.5.txt

@@ -557,6 +557,15 @@ config files, e.g. "repo.desc" becomes "desc".
 
 FILTER API
 ----------
+By default, filters are separate processes that are executed each time they
+are needed.  Alternative technologies may be used by prefixing the filter
+specification with the relevant string; available values are:
+
+'exec:'::
+	The default "one process per filter" mode.
+
+Parameters are provided to filters as follows.
+
 about filter::
 	This filter is given a single parameter: the filename of the source
 	file to filter. The filter can use the filename to determine (for