Provide a way to disable treeShaking

[matthewp]

This makes it possible to disable tree shaking 2 ways:

<script src="./node_modules/steal/steal.js"
  main="~/app"
  no-tree-shaking></script>

This allows disabling it on a page-by-page basis. You can also use the package.json flag:

{
  "steal": {
    "treeShaking": false
  }
}

The above is only respected from the root package.json. Closes #1433

[matthewp]

This isn't quite ready to be merged yet.

[leoj3n]

How can I verify from within the built files that treeShaking has indeed been disabled?

[matthewp]

Check if something you expected to be treeshaken was.