Add option to keep all attributes as strings

[RichardCzechowski]

When data is coming from user supplied HTML, I would expect all attributes to come in as strings. Instead this library converts boolean strings "true" and "false" to boolean primitives. My use-case is very heavy on custom components, many of which do string operations on the incoming data. For example we have one that passes in a a path to get properties off of an object, that path can be the string literal values "true"/"false". It would be nice to be able to toggle off the feature that converts booleans.