Disable focus trap & disable tab trap

[bvellacott]

Fixes #[issue number].

Changes proposed:

• add ability to disableFocusTrap
• add ability to disableTabTrap

Acceptance Checklist:

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

[coveralls]

Coverage decreased (-0.3%) to 84.777% when pulling afada57e55e5752983fc29dfb53a97cdbaeff24b on bvellacott:disableFocusTrap-&-disableTabTrap into 827796d48e7d4c74b4362cf90955e162082ee46d on reactjs:master.

[bvellacott]

https://github.com/reactjs/react-modal/issues/799