pg_regress config

I would like to add an argument to pg_regress that allows us to set some
config options for the temp install. Specifically right now I am
interested in setting the following:

log_line_prefix = '[%c] '
log_statement = 'all'
log_connections = 'true'

But I think it makes more sense to provide for a general option setting
mechanism, such as --extra_config=/path/to/some/file

This would dovetail neatly with the recent suggestion that we might
provide for buildfarm members to run several config sets.

Thoughts?

cheers

andrew