[program_options] Allow unrecognized options in configfile

[ensc]

It would be nice when the config-file parser would have
a similar mechanism to allow and collect unregistered
options like the command-line parser.

E.g. with the command-line parser I can use the
allow_unregistered() method, but config-file parser
throws always a boost::program_options::unknown_option
exception.

I want this feature e.g. to express things like

| module = A
| module = B
| [A]
| foo = X
| [B]
| bar = Y

where the options in the [A] and [B] sections are
unknown at the first pass.