Support for external widgets and jobs

[davefp]

This issue covers adding support for bringing in external widgets and jobs (maybe even dashboards?) using packages.

The desired workflow would be something like this, similar to how external scripts are used in Hubot:

1. Add the package as a dependency: > yarn add handsome-custom-widget
2. Add the package name to a file like external.json:

# external.json
[
  'handsome-custom-widget',
  'handsome-another-custom-widget'
]

3. Any widgets and/or jobs from the package are now available when building dashboards.

Proposed External Package Structure

External packages should have a /widgets folder that contains any widgets they want to add. Similarly, they should have a /jobs folder that contains any jobs they want to add.

Open questions

• How are external jobs configured (interval, usernames, etc.)?
• Should all jobs from a package be run automatically once the package is added or should they need to be configured further first?