Provide addCollection API method, add to application.collections Map

[paulrobertlloyd]

• Adds a new addCollection API method. Fails if adding a collection with a name that already exists.
• endpoint-micropub plug-in takes responsibility for adding the posts collection
• endpoint-media plug-in takes responsibility for adding the media collection
• Indiekit instance tidied up a bit, moving setup out of bootstrap() and adding the properties cache, database and the following methods: connectMongodbClient() and closeMongodbClient()
• Collections added to Indiekit.collections Map (exposed via Express locals as application.collections).