[added] isNested prop

[Jarch09]

Changes proposed:

• adds isNested: bool (optional) prop, which gives you the option of not removing the modal backdrop.

Why? This enables Russian doll nesting, so you can have multiple modals open on top of each other.

Acceptance Checklist:

• 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 (+0.06%) to 86.758% when pulling 4c17fc4 on Jarch09:add-nesting into 1b561fc on reactjs:master.

[diasbruno]

Hi @Jarch09, I'll have a look on all PRs this weekend. Thanks!

[Jarch09]

Hey you have a chance to check this out?

[diasbruno]

@Jarch09 Yep. This looks good. I'm thinking of push this to v4.

[willsmanley]

So is this going to be pulled at any point?

Does this change only remove the background effect or does it also affect modal stacking order?