Add runway visible range support

[dmbaturin]

Request from a user:

I was trying to decode METAR using PyTaf library. The library is very useful and decodes very well except RVR.

For example : METAR VOML 101430Z 28005KT 1000 R24/1200 +TSRA BR FEW004 SCT012 FEW025CB BKN080 26/22 Q1009 NOSIG=

The library ignores  R24/1200. Requesting you to kindly incorporate RVR decoding in PyTaf.