Implement SPS parsing

[jaredwsmith]

Special Weather Statements and Significant Weather Advisories. NWS Charleston, SC does not issue significant weather advisories by name, but many other offices do, so we'll work a little support in.

SPSes can be polygonal or zone-based depending on how they are issued. We should interpret polygonal SPS products in the “significant weather advisory” column. You can also see SPS products for full forecast zones.

Recently, impact-based bullets have made it to the SPS/SWA, so we can perhaps parse those out, too.