Document how to test a preset

[drzraf]

Problem description:

plugin and pluginName are plugin-centric. I would like to test a preset which bundle a couple of plugins.

Suggested solution:

Could you please document how to do that (if it's possible)?

[kentcdodds]

This isn't directly supported, but it could be (I don't think it would take too much work). Would you be interested in making a pull request to add this feature?

[Xunnamius]

I'm thinking a new set of options: preset, presetName ~~(required)~~, and presetOptions. This set of options would be mutually exclusive with the counterpart set: plugin, pluginName, and pluginOptions. If any of the options from one set are used, options from the other set cannot be used.

From there, it's just a matter of wiring up [preset, presetOptions] to be injected into the final babelOptions (babelOptions.presets rather than babelOptions.plugins) and using presetName ~~(which would be non-optional)~~ in the describe block instead of pluginName. Given #91, it'd be prudent to export a new symbol for supporting an arbitrary run order of the preset under test too.

I'll work on a PR for this.