expose modifier utility function to consumer

[cwmoo740]

Great library, by the way.

I'm working on a project that needs to validate dates based on these modifiers (user can either type into a text box, or use the react-day-picker). It would be nice if I could import a function that lets me validate if a react-day-picker modifier applies to a given date.

This is one proposal here - it doesn't add very much code, just shrinks the Helpers.js and splits up getModifiersForDay into two functions.

[coveralls]

Coverage remained the same at 100.0% when pulling ab2b4c9 on cwmoo740:master into d12c4af on gpbl:master.

[gpbl]

This is great, thanks! I was thinking to something similar 👍

[gpbl]

Releasing this later in few hours :)

[gpbl]

(I meant "few hours" as here in Peru, where it means days later 😆)

Actually I was in the middle of moving to CircleCI, and I've had to add the docs (see here).

The utilities are published in v5.4.0, please note that the file is called ModifiersUtils (with Modifiers, plural) instead of ModifierUtils (singular).

Please report back if something is not working as expected, thanks!