Support for includes

[daniele-athome]

References are relatively easy, the main issue will be how to identify this:

configuration.yaml:

automation: !include automations.yaml

automations.yaml:

- alias: automation_one
- alias: automation_two

without building a complete model of all included configuration files, how do I check that those sequence items are actually children of automation? The current code analyzes the YAML tree per file.

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The IntelliJ YAML plugin seems to be recognizing YAML anchors, but in our case YAML tags as used as anchors. We'll have to implement something to handle those.

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This will hardly make into 1.0: it turns out it is more difficult than it seemed. As a first step, I might recognize in a "special way" the UI-managed YAML files (automation.yaml, scripts.yaml).