A 'build application' task

[jespertheend]

A task that has a minimal amount of configuration and should 'just work'. This task is meant for users that don't need a lot of control over the build process.
The task is expected to take a single asset as entry point, this can either be an entity or a script file.

• If a script is provided. The 'bundle assets', 'bundle scripts', 'generate html', and 'generate services' tasks are used for building the application.
• If an entity is provided, there will be a minimal amount of interactivity in the built application, all the same steps are performed as for script entry points, except a little bit of extra script is generated to initialize the services and add a canvas to the page.

Depends on:

• Reusable environment variables across tasks. #62
• Support making tasks dependent on other tasks #9

[jespertheend]

Change of plans: we'll only allow a script as entry point.
If the user wishes to build an application with only an entity as entry point, we can make a separate task type for that.
Since in that case you'll likely want to be able to have more configuration options.