Throw err when the second arguments is not a Number/String

[LeandroEstevez]

Throw err when the second arguments is not a Number/String

How it is now:

import m2r from 'minutes-to-read';

m2r('str', {} || []);

Nothing will happen

import m2r from 'minutes-to-read';

m2r('str', {} || []);

Now it will throw an error

Expected an String/Number in the second argument, got {something else}