Added support for functions as validation message props

[BowlingX]

Thank you so much for this library.
This pull request will add the ability to specify functions as message props:

Example

<Html5ValidationField
  tooShort={(value, { minLength }) => `Value ${value} should have at least ${minLength} characters.`}
  minLength={8} 
  name="foo"
/>

This is especially usefull when defining default translation messages for all fields (here with react-intl):

import { injectIntl, defineMessages } from 'react-intl'

const messages = defineMessages({
   valueMissing: 'Value missing',
   minLength: 'Value should have at least {minLength} characters',
   /*... more messages ...*/
})
const defaultMessages = {
  valueMissing: (_, { formatMessage } ) => formatMessage(messages.valueMissing),
  minLength: (_, { formatMessage, minLength } ) => formatMessage(messages.minLength, { minLength }),
/*... more messages ...*/
}
const FieldWithDefaults = injectIntl(({ intl: { formatMessage }, ...rest }) => <Field formatMessage={formatMessage} {...defaultMessages} {...rest } />)

Currently this requires to create an object on each render with default messages based on the Field's props. I think this would provide a more elegant and performant way.

[codecov]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #12   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           2      2           
  Lines          36     38    +2     
  Branches       16     17    +1     
=====================================
+ Hits           36     38    +2

Impacted Files	Coverage Δ
src/Html5ValidationField.js	100% <100%> (ø)	⬆️

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[BowlingX]

Anything else you require for merging / checking this PR?

[BowlingX]

@erikras This pull request is already quite long in the pipeline, do you request any more comment / feedback, tests etc.?

[erikras]

Published in v1.1.0.