Add -x to opt into experimental file formats

Add -x to opt into experimental file formats

Instead of burning one letter for each new file format (sidrops is known
to crank out new things faster than a normal person can read), use -x to
opt into parsing and processing file formats that aren't yet considered
stable. This is currently only the Signed Prefix List.  While a repetition
of the ASPA debacle, this code hasn't yet seen enough stress testing to be
enabled by default.

ok claudio job