Generates unique filename.
Returns uuid. Appends ending if supplied.
const unique = require('tfk-generate-unique-filename')
console.log(unique('.pdf')) //=> ee0c1878-5810-4682-b77b-30f2875ae460.pdf
console.log(unique()) //=> 6d26360b-e0b7-4953-96ed-4004a2f47454