fix for bootstrap@4.0.0-beta

[ghost]

added overrides for bootstrap@4.0.0-beta.
added popper.js to registry.

[aluanhaddad]

Greetings. Thank you for the PR!

Unfortunately, it appears that bootstrap expects Popper to be available as a global. It throws a very specific error if that is not the case.

This means there will need to be an override for popper as well.
The following should work:
package-overrides/npm/popper.js@1.0.0.json

{
  "main": "dist/umd/popper",
  "shim": {
    "dist/umd/popper": {
      "exports": [
        "Popper"
      ]
    }
  }
}

[aluanhaddad]

In previous overrides for bootstrap the SASS files were included, for those who work directly with the source, for example by overriding bootstrap mixins.

To preserve this use case it would be best to remove the "directories" property and instead qualify "main" with dist/.

[aluanhaddad]

It looks like the next beta release will include Popper.js in the bundle itself which will make this easier.