add weather plugin using wunderground API

we ask for weather forecast using resort location and then simplify the
response saving only the forecast for the closest period

only enabled if valid wunderground API key is configured and passed as
`WUND_API_KEY` environment variable
add limiter to prevent overuse of weather APIs
add client-side weather plugin
Merge branch 'weather'
add twitter handles to resort data
add twitter plugin

request tweets for resorts with twitter handles configured

twitter data sent to the client and saved in the cache contains a
twitter handle and up to 3 recent tweets
add client-side twitter plugin

render tweets and use entities to analyze tweet content and add links
and images based on it
Merge branch 'twitter'