Add wind gusts to parsed METAR data

[dopplershift]

Is there a way to output a wind gust value when parsing METARs with metpy.io.parse_metar_file? The documentation only shows output columns for standard wind speed and direction. For example, a METAR with the wind section "12012G18KT" would have a wind gust value of 18 knots.

Originally posted by @DougASchu in #2074

[dopplershift]

Should be easy to add a new column wind_gust alongside wind_speed. The field wind.gust is already coming out of the PEG parser.