feat(weather_hum_): Add a humidity plugin for US NOAA weather

This adds a plugin for relative humidity, based on the weather_press_ and
weather_temp_ plugins. It's basically a copy of these two plugins with only
minor adjustments.

plugins/weather/weather_hum_

@@ -0,0 +1,42 @@
+#!/usr/bin/env python3
+"""
+munin US NOAA weather plugin (http://tgftp.nws.noaa.gov)
+
+Draws relative humidity in %.
+Copy/link file as 'weather_humidity_CODE', like: weather_humidity_LOWW for Austria, Vienna.
+
+Get the code by going to http://tgftp.nws.noaa.gov, selecting your
+location, and copying the code from the address bar of your browser; should
+be something like CODE.html.
+
+Linux users might need to adjust the shebang.
+"""
+
+import sys
+from urllib.request import urlopen
+import re
+
+url = 'https://tgftp.nws.noaa.gov/data/observations/metar/decoded/%s.TXT'
+
+re_hum = re.compile(r'Relative Humidity:\s*(\d+)%')
+
+
+code = sys.argv[0][(sys.argv[0].rfind('_') + 1):]
+if not code:
+    sys.exit(1)
+elif len(sys.argv) == 2 and sys.argv[1] == "autoconf":
+    print("yes")
+elif len(sys.argv) == 2 and sys.argv[1] == "config":
+    print('graph_title Relative humidity at code %s' % code)
+    print('graph_vlabel humidity in %')
+    print('graph_category sensors')
+
+    print('humidity.label Humidity')
+    print('graph_args --base 1000 -l 0')
+else:
+    u = urlopen(url % code)
+    txt = str(u.read())
+    u.close()
+
+    hum = re_hum.findall(txt)[0]
+    print('humidity.value %s' % hum)