Add type definitions, please?

[ajvincent]

TypeScript complains:

Could not find a declaration file for module 'gulp-prettier'. '.../node_modules/gulp-prettier/index.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/gulp-prettier` if it exists or add a new declaration (.d.ts) file containing `declare module 'gulp-prettier';`ts(7016)

This really just needs an index.d.ts file which package.json refers to using the "typings" key.

The contents of this file I do not know off the top of my head. I know how to write TypeScript type definition files. I just haven't worked with NodeJS streams very much.