Function Equal to Compare Strings

Sometimes is very usefull a simple method to compare an String.
It's very simple to implement this in your code, but if the Joi maybe can verify this if I expected a value ever, why not use the power of Joi to return an error when the string is mismatch? 

In many cases I need to compare a string, I sent this PR because for me it's very useful, I hope it will be helpful to others also.

Thanks!