Manually invoked currency refresh

[tiffany352]

Adds currency.fetch_on_startup to the config, which when set to false behaves as if the cache_duration was infinite. This means rink will fetch the file once, and then keep reusing that file indefinitely.

Adds a --fetch-currency CLI argument, which will make rink download the latest version of the currency data and then exit. This can be put into a cron job and used together with the config option so that rink will never block on a web request at startup, without giving up currency units. Requires manual setup though.

[codecov]

Codecov Report

Attention: Patch coverage is 90.12346% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 81.63%. Comparing base (6e674f0) to head (4dba69d).

Files	Patch %	Lines
cli/src/config.rs	90.12%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   80.81%   81.63%   +0.82%     
==========================================
  Files          42       42              
  Lines        7718     7870     +152     
==========================================
+ Hits         6237     6425     +188     
+ Misses       1481     1445      -36     

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