FR: Add util function "encodeAsFirebaseKey"

[greenido]

I keep copy/paste this util function in every project I'm doing...
It might be useful to other developers as it's a common task to create a proper key out of data like: email, phone, name etc'.

Relevant Code:

function encodeAsFirebaseKey (s) {
  return s.replace(/%/g, '%25')
    .replace(/\./g, '%2E')
    .replace(/#/g, '%23')
    .replace(/\$/g, '%24')
    .replace(/\//g, '%2F')
    .replace(/\[/g, '%5B')
    .replace(/\]/g, '%5D');
}

[hiranya911]

We don't expose this type of data cleaning operations in any of our SDKs. Different developers may wish to handle these cases differently, and there's no meaningful way to enforce a global policy in the SDK with regard to these.