GeoIP module not compliant with RFC 2782 / Add "_" (underscore) to yaml parser

SRV resource records as defined in RFC 2782 make use of underscore for the service name and protocol. However, the yaml parser used in Knot doesn't allow underscore in the keyname.

I've played around a bit and changed the key_name variable in ypbody.rl to allow it.

changed (libknot/yparser/ypbody.rl line 144)

key_name = ((alnum | [\\.] | "*." ) . (alnum | [\\.\-])*) >_key_init $_key;

to

key_name = ((alnum | [\\._] | "*." ) . (alnum | [\\._\-])*) >_key_init $_key;

I don't know if this is the best approach to achieve RFC 2782 compliance since I'm not really familiar with C/C++ or Ragel.