[FEATURE] Cache results to avoid multiple requests

[tchar]

Hello,

I have implemented my own currency converter with an inspiration from this extension so that it caches results in order to avoid multiple requests and also speed up the results.
I thought maybe I can adapt my code to yours and make a pull request to support this feature.

• There is an option in the preferences for
  • No caching, 1 day, 1/2 weeks, 1 month, 1 year
• Allow multiple results like "10 EUR TO USD,CAD,AUD"
• Fetch the 10 most common currencies (if caching enabled) in one request and store them for future use so no further requests are made.

Let me know if you are interested in something like that.

Cheers,
Here is my repo https://github.com/tchar/ulauncher-currency-converter