ENH: Implemented Autocorrect flag on numerical fields

I'm not sure if this functionality exists in enaml already as I didn't see it anywhere.

IntValidator and FloatValidator have a 'fix_invalid_input' attribute that controls whether the user input is corrected based on the 'minimum' and 'maximum' of those fields.

The file /enaml/examples/stdlib/numerical_fields.enaml demonstrates this utility.

Another architectural option would be to subclass `IntField` to something like `ValidatedIntField` and have the validation done in the subclass with the same concept being done to `FloatField` and `ValidatedFloatField`.  

Any preferences?