Implement 'register app' functionality

[dmitrizagidulin]

profile.loadAppRegistry() is already implemented. Now add support for registering an app, something like:

var options = {
  name: 'Contact Manager 3.0',
  shordesc: '...',
  // See other fields in https://github.com/solid/solid/issues/109
}
var types = [  // list of types / RDF classes that this app will handle
    vocab.vcard('AddressBook'),
    vocab.solid('Contact')
]
var isListed = true
var app = new AppRegistration(options, types, isListed)
profile.registerApp(app)
  .then(function (updatedProfile) {
    console.log('The app has been registered.')
  })
  .catch(function (error) {
    console.log('Error registering app:', error)
  })

This will create an entry in the corresponding app registry (in this case, the listed (public) app registry).
See solid/solid#109 for additional specs.

To query the app registry:

profile.appsForType(vocab.vcard('AddressBook'))
  .then(function (registeredApps) {
    // registeredApps ->  [ registration1, registration2, ... ]
    var app = registeredApps[0]
    app.name   // -> 'Contact Manager 3.0'
    app.homepage  // -> 'https://example.com/...'
    app.icon  // -> 'https://...'
    // Most importantly:
    app.redirectTemplateUri   // -> 'https://solid.github.io/contacts/?uri={uri}'
  }

The idea being - if multiple apps are registered for the same type, we can present the user with an 'Open with...' screen allowing them to choose between the registered apps.

[dmitrizagidulin]

Implemented. closing.