feat(Modal): Allow passing in an element selector to append modal to

[ramoncaldeira]

• Bug fix
• New feature
• Chore
• Breaking change
• There is an open issue which this change addresses
• I have read the CONTRIBUTING document.
• My commits follow the Git Commit Guidelines
• My code follows the code style of this project.
• My change requires a change to the documentation.
• • I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Refers to #859.

I believe this feature is important when using Fullscreen API to request fullscreen for an element other than body. Specifying another parent for the modal allows the Modal to be used in fullscreen mode.

I couldn't add more tests because of a limitation of Enzyme to test portals.

[TheSharpieOne]

nice. One nit, can you rename it to container since there is an existing prop for other components that does the same thing.

[ramoncaldeira]

@TheSharpieOne Sure. I updated the PR with this change