Types/Typescript support

[james-pre]

Currently, Postject does not make types available for the Javascript API, so it is unwieldy to use with strict Typescript. Strict Typescript gives an error when importing from postject.

Could not find a declaration file for module 'postject'. '<NPM project root>/node_modules/postject/dist/api.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/postject` if it exists or add a new declaration (.d.ts) file containing `declare module 'postject';`ts(7016)