create netlify.toml during netlify init

**- Do you want to request a *feature* or report a *bug*?**

feature

**- What is the current behavior?**

no netlify.toml on netlify init

**- What is the expected behavior?**

scaffolds out a basic netlify.toml on netlify init

discussed in netlify internal slack https://netlify.slack.com/archives/CBC2U9MMG/p1552930811050500
