Start attributes

The parser won't start the process unless it find an attribute that implements IStartAttribute (which is internal in 1.1.0, but will be public later on).

Start attribute has to be placed at the top of property's attributes and there can be only one start attribute on one property.

Start attribute receives actual* root node of the document and performs it's operations.

The start attribute can set its result to property.

*If e.g. WebsiteParserModel has set selector, the root node will be selector inner html, not the DocumentNode