Allow configuration of auto launch feature

[RaeesBhatti]

- Summary

Some users have a requested a way to disable the feature in Netlify Dev that automatically launches the local dev server in browser. This change implements a way to disable that feature from netlify.toml. Fixes #615

- Test plan

Add the following to your netlify.toml

[dev]
  autoLaunch = false

Run netlify dev

- Description for the changelog

Check for config.dev.autoLaunch to determine if we should launch the browser automatically or not.

- A picture of a cute animal (not mandatory but encouraged)
🐥

[ianmartorell]

With this change, the default behaviour changes to not auto launching the browser. This is an unexpected change and might break some development environments.

For instance, I just opened #672 to auto detect Expo projects, but I need the browser to be auto launched because the Expo cli clears the terminal when running, making the netlify-dev URL disappear, and we can't rely on the user to add the autoLaunch = true flag to their netlify.toml. Ideally, auto-detected servers should work out of the box.

So the way I see it, one of these should be done before the next release:

1. Revert default behaviour to autoLaunch = true
2. Check autoLaunch in the auto-detected server settings too

Ideally it would be the first one, to avoid changing default behaviour. I'm happy to open the pull request.