Add support for passing metadata into visitOptions of fastboot visit

[suchitadoshi1987]

Add support for passing metadata into visitOptions of fastboot visit

Context:
Currently, with local development of addons that rely on fastboot metadata, there are cases where the metadata is set by another library of the consuming app. Hence, local development of such addons can become challenging. This PR introduces a way to pass in the metadata through the visitOptions, so that the dummy apps could leverage it.
Once this change is in, we could potentially pass in the metadata into the middleware from here:
https://github.com/ember-fastboot/ember-cli-fastboot/blob/master/packages/ember-cli-fastboot/index.js#L339.

Ref: https://github.com/rileyhilliard/fastboot-addon-development
TODO:

• Add Tests

[rwjblue]

In general, this seems fine but I'd prefer to change this a bit so that we grab options.visitOptions instead.

Then we can tweak the thing in the returned function to be something like:

let visitOptions = Object.assign({}, opts.visitOptions, { request: req, response: res });
fastboot
  .visit(path, visitOptions)
  .then(success, failure);

[rwjblue]

No giant rush, but we should also make sure to add some docs to the README about this before we land it.