Appium 2.0 Recommendations

## Summary

The [proof of concept](https://github.com/dpgraham/appium/tree/dpgraham-monorepo) is a monorepo with [Lerna](https://lernajs.io), where the repo has scoped NPM packages that export drivers (`@appium/xcuitest-driver`, `@appium/espresso-driver`, etc...). The package `appium` is a CLI that installs and manages these drivers.

## Description
* Appium 1.x imports and bundles together all drivers. The PoC breaks Appium into different packages.
* One package is `@appium/builder`, which does almost exactly what `appium/appium@1` does, except instead of running the Appium server, it exposes an API that is used to build and run an Appium server.

```
//example usage
new AppiumBuilder()
  .withAutomation('appium-xcuitest-driver', XCUITestDriver)
  .withAutomation('appium-espresso-driver', EspressoDriver)
  .run()
```

* The package `appium` is a CLI that installs drivers, uninstalls drivers, etc... (all the things discussed in the proposal). It uses `@appium/builder` to take the installed drivers and run them. The drivers are installed to a directory that is kept on the user's filesystem.

```
// appium/lib/index.js
#!/usr/bin/env node
// transpile:main

module.exports = require('@oclif/command');

import { asyncify } from 'asyncbox';
import path from 'path';
import AppiumBuilder from '@appium/builder';
import _ from 'lodash';

async function main (args = null) {
  const drivers = require(path.resolve('appium-drivers', 'drivers.json'));
  const builder = new AppiumBuilder();
  for (let [name, packageInfo] of _.toPairs(drivers)) {
    const packageName = packageInfo.package;
    const driver = require(path.resolve('appium-drivers', 'node_modules', packageName)).default;
    builder.withAutomation(name, driver);
  }
  return await builder.run(args);
}

if (require.main === module) {
  asyncify(main);
}

export { main };
```

## Versioning and publishing
* The monorepo approach with Lerna allows one-stop versioning and publishing. The drivers can all be published with one command `lerna publish` which publishes the packages in the repo, and only publishes packages that have changed since the last time they were published. 
* We can keep doing Appium versioning via Git tags (2.0.0, 2.1.0, 2.1.1, ...) and use the semver-esque versioning schema that we're currently using
* The packages will be published to NPM and use strict semver versioning

## Packages
* `appium`: The CLI that installs and manages the drivers. Uses `@appium/builder` as a dependency to run the drivers.
* `@appium/builder`: A package that is used to build Appium servers
* `@appium/xcuitest`: A package that exports `appium-xcuitest-driver`
* `@appium/uiautomator2`: A package that exports `appium-uiautomator2-driver`