Add support for PAR/PAR2 records

[ci-macpaw]

Originally reported on Google Code with ID 39

paracelsus:

There's no support for PAR/PAR2 records. This could theoretically be added,
but currently I am not sure I have the time. Therefore, any external help
would be very much appreciated.

For anyone interested in helping out with this: libxad reads data through a
custom callback handler, that implements open/read/seek/close
functionality. Multi-part archives are handled transparently through this,
for instance. A PAR/PAR2 handler could be written that works like the
multi-archive handler but additionally fills in any missing pieces by itself.

[ci-macpaw]

Stéphane Moureau (TraderStf):

TraderStf:

several open source available, explanation of the difference, formats... that would
be a nice feature, there is no solution on mac for these kind of files.

[ci-macpaw]

jarvik7:

I don't suppose that you would implement SFV support in addition to this, since SFV
has no output file...
It would be nice to ditch some other single-purpose utils.