add attribute label support

[uiii]

Hi, I've implemented support for attribute label, which when used is printed in the error messages instead of "pretiffied" attribute name (see https://github.com/ansman/validate.js/issues/69#issuecomment-194771733). I have updated tests, but maybe didn't cover the code completely. I didn't updated doc. Finally I've used another validation library, so I don't rely on this anymore, but I don't want throw away the work. If you found my implementation useful, here it is, if not, just delete the PR.

[ansman]

Thanks, I'll take a look at this later.

[michaek]

This seems to be an improvement on overriding the prettify function in the sense that it's scoped to the options argument of a validate() call, but it's limited to matching field names exactly to make replacements.

[thg303]

please resolve this conflict. I need this feature

[orditeck]

@ansman any update on this? Would be a nice feature

[mindaugasnakrosis]

@uiii Whst is the status?

[uiii]

@mindaugasnakrosis This is quite old, read the description, I think it explains it enough. I don't use this library anymore.