Introducing the support for the `yield` context parameter.

Without specifying the (new) config parameter: `LEGACY`, Micro will work slightly different. Micro will trigger the page Controllers and it will render the View for the given `request path` *before* merging it with the Template View. So you don't have to pull the View in your main Template anymore. The result of this process will be available as a new context parameter named: `yield`

Before this change (see code tagged 0.1.2), your default Template had something like this (using Velocity syntax):

    $content.get($path)

If you want to maintain the "legacy" style, make sure you set LEGACY: true, in your micro-config.yml file.

The next Micro versions will use the new style, therefore the refactored template above will use this instead:

    $yield