Adds the ability to report based on PR comments

[LeZuse]

In combination with https://github.com/Khan/pull-request-comment-trigger you can completely customize when to run WPT on a PR.

Example:

on:
  pull_request:
    types: [opened]
  issue_comment:
    types: [created]

jobs:
  build:
    runs-on: ubuntu-latest
    name: WebPageTest Action
    steps:
      - uses: khan/pull-request-comment-trigger@master
        id: check
        with:
          trigger: '/wpt'
          reaction: rocket
          prefix_only: true
        env:
          GITHUB_TOKEN: '${{ secrets.GITHUB_TOKEN }}'

      - name: Checkout
        if: steps.check.outputs.triggered == 'true'
        uses: actions/checkout@v2
        
      - name: WebPageTest
        if: steps.check.outputs.triggered == 'true'
        uses: productboardlabs/webpagetest-github-action@pr-comment-trigger
        with:
          apiKey: ${{ secrets.WEBPAGETEST_API }}
          urls: |
            http://example.com
          label: 'GitHub Action Test'
          wptOptions: 'wpt-options.json'
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          

[tkadlec]

This looks great, @LeZuse! Thanks for adding this.