New app creation process

[nicolaskruchten]

For Dash, setting up an empty repo to create a new app involves pip-installing 3 separate packages.

For parity, DashR should aim for something similar: 3 install-from-github calls (DashR, HTML and Core components). Because DashR itself will be a private repo for a little while, this will rely on the GITHUB_PAT environment variable being set, per https://www.rdocumentation.org/packages/devtools/versions/1.13.6/topics/install_github

This issue can be closed when the README for DashR documents this new process and it works :)

In addition, all of the examples and docs in this repo should work when the new installation process is used. They'll likely need to be adjusted to capture the removal of dependencies from DashR to HTML/Core components, and then QA'ed to ensure that they all work using the latest version of the components.

Depends on plotly/dash-html-components#74 and plotly/dash-core-components#357

[rpkyle]

This is now complete. 🎉