Add Jira support

[mbland]

This will require:

• adding a new JiraClient class, with slight updates to the existing GithubClient interface to match a new IssuesClient interface
• updating Config to encapsulate the github* settings, and create a corresponding set of jira* settings
• updating Rule to change githubRepository to issuesBoard or some such
• adding an IssuesClientFactory or some such to instantiate the correct issues client in index.js, based on the validated Config

References:

• JIRA Cloud REST API Reference
• JIRA REST API Example - Create Issue

[mbland]

While beginning to do some refactoring to make the existing code more generic, I realize that the current Config.githubUser can not only become Config.collection, but that Rule.target doesn't have to be a string: for the JIRA implementation, it can be an object directly injected into the API request body.

At the same time, Config.collection can become the default GitHub org/user or default JIRA project ID/key, which can then be overridden by individual Rule.target objects.