[added] Allow custom Modal dialog components

[jquense]

pulled out from #1040, needs some additional bike shedding

cc @mfunkie

[taion]

Reprise - the question to me is whether we like the current API better, or if it would be better to do e.g.

<Modal customDialog={true} ... >
  <MyModalDialog />
</Modal>

[AlexKVal]

I vote for @taion proposal:

<Modal customDialog={true} ... >
  <MyModalDialog />
</Modal>

I'm proposing the same API for another issue: #1071

There are two versions of API you can compare #1071 and #1072

[AlexKVal]

Matt also is a proponent of such API 😄 #748 (comment)

[taion]

I retract what I said earlier.

For handling the "generic modal" use case, it makes more sense to split out a new package from this with the core utilities around e.g. Modal, Overlay, Transition, Position, and friends.

It makes more sense for the Modal component in React-Bootstrap itself to map well to the Bootstrap modal, which this accomplishes quite well, while still giving some flexibility for customization.

I'm okay with merging this as-is. Will open a new issue regarding pulling out core components.

[taion]

@jquense Can you rebase this? Let's merge this and defer the generic API to #1084, unless there are further objections.

[taion]

I rebased - the conflict was spurious (some overlapping lines with another added test).

[jquense]

I am fine with this as is if you are.