Add support for PMTK_LOCUS_CONFIG logging configuration

Add support for configuring logging using the PMTK_LOCUS_CONFIG ($PMTK,187) sentence, along with dynamic checksum configuration using code references here https://stackoverflow.com/questions/32076761/nmea-checksum-calculation-calculation

You can manually compute and send the PMTK_LOCUS_CONFIG sentence now, but library support would be nice.