feat: add custom convertCurrency function

[JB1905]

Closes #33
Closes #39

[codecov]

Codecov Report

Merging #53 (7ad11af) into master (f99c310) will decrease coverage by 3.33%.
The diff coverage is 96.00%.

@@             Coverage Diff             @@
##            master      #53      +/-   ##
===========================================
- Coverage   100.00%   96.66%   -3.34%     
===========================================
  Files            3        3              
  Lines           33       30       -3     
  Branches         8        7       -1     
===========================================
- Hits            33       29       -4     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
src/useCurrency.ts	96.15% <96.00%> (-3.85%)	⬇️

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