Add ability to use failsafe scheme - only maps, seqs and strings

[Mingun]

It would be nice to have the ability do not interpret strings as numbers and bools and everything return them as strings. In YAML 1.2 this is called failsafe schema. I known, that currently this project targeting YAML 1.1, but even in that case the ability to not interpret input would be useful. Also, I think, that should be easy to implement