Allow automated screenshots

If the repo as a gramex.yaml with a test.url section, monitor acts on these.

test:
  config:
    base_url: https://learn.gramener.com/guide/   # URL where the app is deployed. Variable name TBD
    email: s.anand@gramener.com                   # Where to email screenshots. Defaults to committer. Variable name TBD
  url:
    - /formhandler/                       # checks for status=200
    -
      url: /filehandler/
      headers:                            # Allow custom HTTP headers
        User-Agent: ...
    -
      url: /r/modelhandler?error
      status: 500                         # Test that the HTTP status is 500
    - 
      url: /path/
      screenshot: true                    # Email the screenshot to the user who pushed
    -
      url: https://learn.gramener.com/guide/2
      screenshot:                         # Pass additional parameters
        width: 300
        height: 300