Implement keys and endkeys (like GoValidator) #48
Description
After #7 finished, it's necessary a way to validate the keys in a map field.
GoValidator uses the keys and endkeys to determine when a validation must be executed on the keys (default is on the values).
From GoValidator docs:
"These are to be used together directly after the dive tag and tells the validator that anything between 'keys' and 'endkeys' applies to the keys of a map and not the values; think of it like the 'dive' tag, but for map keys instead of values. Multidimensional nesting is also supported, each level you wish to validate will require another 'keys' and 'endkeys' tag. These tags are only valid for maps."