Add Swap feature to CurrencyInput

[chikeichan]

[danjm]

Unit test and lint fixes needed

Code looks good otherwise

[bdresser]

that was fast @chikeichan !

@danfinlay @cjeria were you imagining the value would stay the same and the denomination would swap (as in the GIF) or that the values would swap along with the denomination? I can see arguments for either way, just want to check ~

[cjeria]

The currency converter app I use keeps the denomination the same when swapping, so I'd go with how @chikeichan implemented it.

[whymarrh]

LGTM