Feature/specify container

[mpontus]

This PR makes it possible to specify a custom mount node for the portal component.

To conform with conventions established in React.createPortal and MUI container prop has been renamed to rootComponent and reused for this purpose.

• Rename ModalProvider#container prop to rootComponent
• Introduce ModalProvider#container prop to specify DOM element to mount modals into
• Add prop validation logic to ModalProvider to help users transition after the breaking change
• Unexpose ModalRoot to prevent unexpected use-cases
• Update documentation

Fixes #18

[codecov-io]

Codecov Report

Merging #19 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #19   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines          48     57    +9     
  Branches        7      9    +2     
=====================================
+ Hits           48     57    +9

Impacted Files	Coverage Δ
src/ModalRoot.tsx	100% <100%> (ø)	⬆️
src/ModalProvider.tsx	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4e410e0...38bf196. Read the comment docs.