Improve form valid state

[rubencosta]

This PR has the following changes all related with form isValid state:

• updateInputsWithError now takes a second argument that when it evaluates to true causes the form to be invalidated
• passing non empty validationErrors causes the form to be invalidated unless preventExternalInvalidation is set
• updated API.md for updateInputsWithError

I have made this changes because I consider the form to be invalid when the fields have validation errors. I also depend on the form valid state for disabling the submit button while the form is invalid. This is inconsistent due to the submit button being disabled by client side validation but not by server side validation.

Thanks for this amazing library and all the hard work :)

[MilosRasic]

Looks good. Would you mind adding tests to cover this behavior?

[rubencosta]

@MilosRasic I have added some tests, please review.

[rkuykendall]

This looks really solid. Am I reading it right that this would invalidate the form in situations where it currently remains valid though?

[rubencosta]

@rkuykendall yes, passing a non empty validationErrors prop invalidates the form.

[rkuykendall]

@rubencosta Thank you for this PR as well! I'd like to merge it immediately to master.

Same as I said in the other PR, unfortunately, it requires the branch to be cleaned up some. I have a near-term goal of getting these build files out of PRs so they are easier to rebase and merge.

I'll be watching the Gitter chat. I would appreciate your expertise on this project if you're interested.

[rubencosta]

@rkuykendall I rebased the branch on the current master. I'm looking forward to seeing version 2 released 👍
I'll try and drop by the chat :)