fix: export typescript interfaces

[OACDesigns]

exports the typescript interfaces for polished data from the alpha namespace, so they
can be used when imported as an es module.

for example:

import Alphavantage from "alphavantage"
const alpha = Alphavantage({key: 'api_key'});

// ...
const polishedData = alpha.util.polish<Alphavantage.StockQuote>(rawData);

closes #196

[coveralls]

Coverage decreased (-77.0%) to 16.97% when pulling 7561adf on OACDesigns:export-typescript-interfaces into 847f843 on zackurben:master.

[zackurben]

@OACDesigns Thanks for the contribution! My apologies for the delay.

[OACDesigns]

thanks for merging! :)