Added close modal on right click

[ZainGulbaz]

Fixes #837 .

Changes proposed:

• Added feature to close modal on right click on overlay
• Use props shouldCloseOnOverlayRightClick={true} to enable this feature
• By default it is false -Its functionality is same as prop shouldCloseOnOverlayClick <Modal shouldCloseOnOverlayRightClick={true}>

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.

[diasbruno]

@ZainGulbaz Looks good, but I don't think this is the right way to go...

Using a flag is nice, but later on someone will want to bind the context menu and we are going to have a flag and a callback to deal with.

In this case, it'll be better to let the user handle the event and decide want it's going to happen...

<Modal ... onOverlayContextMenu={closeModal} />

[ZainGulbaz]

@diasbruno Ok, I will go with this approach.

[doeg]

@diasbruno I think this PR can be closed out given that we now have https://github.com/reactjs/react-modal/pull/963, which implements onOverlayRightClick per your suggestion (but isn't quite merge-ready).