Add delegate to check if pan gesture should begin

[damienvieira]

Summary

This change allows to control if the pan gesture should begin or not. This is particularly important when your table view supports drag and drop or swipe to delete, since the pan gesture will conflict with the other table view gestures/touches.

The current delegate function func shouldRespond(to panModalGestureRecognizer: UIPanGestureRecognizer) -> Bool it is not enough to manage the pan gesture, because it's called too late during the gesture handling process.

Related issue/question: https://github.com/slackhq/PanModal/issues/82

Requirements (place an x in each [ ])

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.

• [x] I've read and agree to the Code of Conduct.

• [ ] I've written tests to cover the new code and functionality included in this PR.

[CLAassistant]

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[Roach]

@damienvieira would you mind signing the CLA when you have a chance?

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You have signed the CLA already but the status is still pending? Let us recheck it.