Generate a unique character string suitible for use in files and URLs.
var uniqueSlug = require('unique-slug')
var randomSlug = uniqueSlug()
var fileSlug = uniqueSlug('/etc/passwd')
If str is passed in then the return value will be its murmur hash in hex.
If str is not passed in, it will be 4 randomly generated bytes converted into 8 hexadecimal characters.