Add backend for yr.no

[seeba8]

Motivation and Context

https://yr.no is a norwegian website ("A collaboration between NRK and The Norwegian Meteorological Institute") offering weather data with (in my opinion) very usage-friendly API conditions.

Description

I added a backend for the yr.no locationforecast api.
This is the first time I wrote golang code, so please review and let me know where I messed up, I am happy to fix it.

Steps for Testing

./wego -yrno-user-agent "wego CLI, [email protected]" -b yr.no

Screenshots

me@fedora-desktop ~/p/wego (master)> ./wego -d 0
Weather for 48.1,11.6

  _`/"".-.     Rain showers
   ,\_(   ).   12 °C          
    /(___(__)  ↓ 7 km/h       
    ‚ʻ‚ʻ‚ʻ‚ʻ                  
    ‚ʻ‚ʻ‚ʻ‚ʻ   0.6 mm/h       
    ```