[added] allow users to pass aria-* attributes.

[diasbruno]

This patch allows users to pass custom aria attributes
to be added to the modal content.

It accepts an object where the keys are the names of the attributes
without the prefix aria-.

aria attribute is optional.

Usage

<Modal 
   ...
   aria={{
      describedby: "Aid",
      labelledby: "Bid"
   }} />

Changes proposed:

Upgrade Path (for changed or removed APIs):

• None.

Acceptance Checklist:

• All commits have been squashed to one.
• The commit message follows the guidelines in CONTRIBUTING.md.
• Documentation (README.md) and examples have been updated as needed.
• If this is a code change, a spec testing the functionality has been added.
• If the commit message has [changed] or [removed], there is an upgrade path above.

[coveralls]

Coverage increased (+0.2%) to 85.714% when pulling 2bec9e3 on diasbruno:feature/allow-custom-aria-attributes into e5090de on reactjs:master.

[coveralls]

Coverage increased (+0.2%) to 85.714% when pulling 2bec9e3 on diasbruno:feature/allow-custom-aria-attributes into e5090de on reactjs:master.

[coveralls]

Coverage increased (+0.2%) to 85.714% when pulling 2bec9e3 on diasbruno:feature/allow-custom-aria-attributes into e5090de on reactjs:master.

[coveralls]

Coverage increased (+0.2%) to 85.714% when pulling 2bec9e3 on diasbruno:feature/allow-custom-aria-attributes into e5090de on reactjs:master.

[coveralls]

Coverage increased (+0.2%) to 85.714% when pulling 222e708 on diasbruno:feature/allow-custom-aria-attributes into e5090de on reactjs:master.

[coveralls]

Coverage increased (+0.2%) to 85.714% when pulling 222e708 on diasbruno:feature/allow-custom-aria-attributes into e5090de on reactjs:master.

[coveralls]

Coverage increased (+0.2%) to 85.714% when pulling 222e708 on diasbruno:feature/allow-custom-aria-attributes into e5090de on reactjs:master.