Add template support

Created by: mrmckeb

This is a work in progress. Do not merge.

This has become a little more complicated than I first envisioned. Right now, I'm still working through the init part of this. The plan is as follows:

• react-scripts and create-react-app-template-* are installed as dependencies. Either or both of those can be defined via CLI.
  • Default is current default template.
  • Although the supported scripts version (range) is included in the template's peerDependencies, this is not used during install.
• Upon react-scripts init, the template is copied from template package, and peerDependencies are installed from that package (excluding the scripts-version). That package is then removed, as it no longer has any value.
  • When using non-standard scripts, or a scripts version that is below the peerDependencies of the template, appropriate warnings will be shown - but the user can still continue. This, importantly, allows custom scripts versions to work with the template.
  • Optionally, the scripts version range could be in engines... but that doesn't feel clean.

The typescript flag is deprecated under this process.