Validation visibility

[samhinton88]

JIRA

This adds a prop to inputProps that allows a client to pass a function with determines whether or not validation messages should be shown.

This seems to offer a good level of flexibility to clients who want to determine when to show visibility, whilst also maintaining raw validation state behind the scenes and allowing us to provide reasonable defaults.

The library comes with generic and onBlur functions. generic is used by default and will indicate that validation messages should be shown whenever the field is invalid.

An example function:

export const notWhenChanging: VisibilityCallback = ({ 
  validation: { valid }, 
  hadChanged, blurred
}) =>  !valid && blurred && !hadChanged

This could be achieved with top level state, but it seems cleaner to offer this field-level api.

Questions:

• Should the inputState be available to the visibility callback? eg. on .parentState property or similar.