Include fixes.

[SebastianStehle]

This PR contains a wide range of fixes, basically to address issues with include as mentioned or discovered in #177

1. mj-attribute can be inside mj-body

mj-attribute can be inside a mj-body with mj-include. Therefore the child elements can actually change the parent elements as well but because of attribute inheritance parents can also affect the children. Therefore we need 2 passes:

1. Read the whole MJML and store all attributes in binders
2. Make the binding process.

Therefore we cannot reuse the binder anymore, which will reduce the performance a little bit.

2. mj-attribute affects all components

Before we have only handled binded attributes but in a few cases we get the child attributes from a component (see column or section).

This is also resolved because we had to store the binder now where we can solve that easily.

3. Child components of an include must be the child of the parent.

Because of parent-child inheritance we must just ignore that the mj-include even exists. This has been fixed by using the parent of the mj-include when parsing the included content.

I am not sure if mjml and mjml-body in an included file are also dummies that need to be flattened.

4. Simplify validators

We have the full component tree anyway.

5. Allow multiple heads

[LiamRiddell]

Quite the handful of fixes, it's interesting how many edge cases there is with this project.