Improvement: make parameters changeable

[juliansteenbakker]

This PR makes the parameters changeable so users can tweak it to their own needs. The following parameters are available:

    this.decelerateEasing = const Cubic(0.0, 0.0, 0.2, 1.0),
    this.minFlingVelocity= 500.0,
    this.closeProgressThreshold = 0.6,
    this.willPopThreshold= 0.97,

If not provided, they will use the default values.

[juliansteenbakker]

@jamesblasco If you need some help with maintaining this package I'm happy to help.

[TheGlorySaint]

@jamesblasco Could you please look at this PR and merge it?

[jamesblasco]

Sorry for not looking into it before. There have been several PR already that expose some of this parameters so I have exposed the rest in a new PR that did not have so many conflicts . Thanks for the PR 🙏🏻