Adds Danger support

Summary:
Testing Danger support in CI. Continuation of #14964, which Circle stopped building.

Update your node modules first: `npm install`

`npm run danger pr https://github.com/facebook/react-native/pull/14951`
Verify output. This PR should trigger a WIP warning, as well as a package.json warning:

```
> [email protected] danger /Users/hramos/git/react-native
> node ./node_modules/.bin/danger "pr" "https://github.com/facebook/react-native/pull/14951"

{
  fails: [],
  warnings: [
    {
      message: ":construction_worker: Work In Progress - <i>Do not merge yet.</i>"
    },
    {
      message: ":lock: Changes were made to package.json - <i>This will require a manual import. Once approved, a Facebook employee should import the PR, then run `yarn add` for any new packages.</i>"
    }
  ],
  messages: [],
  markdowns: ["This PR requires attention from the facebook/react-native team."]
}
```

`npm run danger pr https://github.com/facebook/react-native/pull/14946`

Verify output. This PR should trigger a warning against the lack of a test plan (note that the PR does have a test plan, but it does not title it as such):
```
{
  fails: [],
  warnings: [
    {
      message: ":clipboard: Test Plan - <i>This PR appears to be missing a Test Plan</i>"
    }
  ],
  messages: [],
  markdowns: []
}
```

`npm run danger pr https://github.com/facebook/react-native/pull/13186`

Should warn against a missing test plan:

```
{
  fails: [],
  warnings: [
    {
      message: ":clipboard: Test Plan - <i>This PR appears to be missing a Test Plan.</i>"
    }
  ],
  messages: [],
  markdowns: [":page_facing_up: Thanks for your contribution to the docs!"]
}
```

If the author is able to issue bot commands, we reasonably assume that this is coming from an established core contributor. Their PRs will be flagged for expedited review:

`npm run danger pr https://github.com/facebook/react-native/pull/14895`

```
{
  fails: [],
  warnings: [
    {
      message: ":clipboard: Test Plan - <i>This PR appears to be missing a Test Plan.</i>"
    }
  ],
  messages: [],
  markdowns: ["This PR has been submitted by a core contributor. Notifying facebook/react-native."]
}
```
Closes https://github.com/facebook/react-native/pull/15061

Differential Revision: D5436605

Pulled By: hramos

fbshipit-source-id: 4ba9e812387d8a69893dab537af9b6cd108753cf