Create should run yarn install automatically.

[mythmon]

Currently after creating a project we ask that users

• cd into the directory
• run yarn install
• run yarn dev

We can't really automate the first and third, but we can automate away the second.

We should attempt to run yarn install (or npm install, as appropriate). This aligns well with other's projects create packages.

We should take care to handle errors here. If the relevant package manager can't be found on the path, or if it runs with an error, we should explain the problem to the user and ask them to run the install command themselves.

[mbostock]

Fixed in #627.