Disable closing modal

A feature request from @bankimatglobant:

Presently the Modal component closes when a user clicks the greyed out background, or the close "x" button. A client developer may want to disable close-on-click-away, and may even want to remove the close "x" (for example in the case of a dialog where use must make an explicit choice).

**Disabling auto-close on click away**

One possibility is to simply expose the existing, separate click handlers on the vanilla modal component.

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Another possibility is to keep the existing generic callback but provide a prop that disables close on click away.

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**Disabling the close "x"**

There may be a valid use case for removing the close "x". Perhaps it could work by adding a prop like this:

```

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Any input @recyclerobot @bankimatglobant?