Cool plugin!

I think a screenshot and a list with supported output options would be helpful,
because with them the future users could immediately understand how cool is the plugin,
and how many possible options it has

get_file_age uses stat -c to get file age, however, the BSD stat does not support -c argument:

stat: illegal option -- c

Maybe add a short paragraph on README.md to instruct MacOS users to have GNU stat installed to make this plugin work?

Relying on IP address to guess the location is often misleading. For example wttr.in guessed based on my IP that I live in Stockholm, Sweden when in fact I live 420 kilometers away on the other side of the country with a completely different forecast than what's shown by the plugin.

Since wttr.in already allows specifying a location with a myriad of options, I see no reason why we shouldn't have that option here too.

In order to change the format, the current instructions read:
set -g @forecast-format %C

This should be changed to:
set -g @forecast-format '%C'