hledger-interest needs some love

[simonmichael]

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hledger-interest seems like it could be useful to many. I think it needs some love to make it easier to understand and use.

http://hackage.haskell.org/package/hledger-interest - home page and nice intro

https://gist.github.com/simonmichael/3fa40418ed75024b30f4 - the example as a shell test, a little easier to grok and tweak