add example support for floats and arrays

[arvidfm]

This PR adds support for the example keyword for floats and arrays. For arrays, it expects a format like:

MyArray []string `jsonschema:"example=A;B"`

[arvidfm]

@samlown I've added tests now

[samlown]

Looks good! Thanks for this! It'd be great to fix parseValue return values if possible.

[arvidfm]

@samlown Sorry for not having gotten around to this until now, but I've now addressed your comments. I adjusted the Type == "" case to accept any kind of value instead of just JSON objects, and added a corresponding test.