Add tag to conveniently insert marker components

[TimJentzsch]

Objective

Closes #3.

Sometimes, we need parts of the text to be updated dynamically, for example every frame.

We can just adjust the BBCode every frame to accomplish this, e.g. using the format! macro, but then we need to parse the BBCode every frame and reconstruct the UI hierarchy.
Needless to say, this doesn't scale very well.

Instead, we want to insert marker components to efficiently query for the Text components we need to change.

Solution

• Add a new [m=foo] tag and BbcodeSettings.with_marker("foo", Foo) syntax, allowing the user to insert marker components on Text entities.
• The marker components can then be used to efficiently update the Text (or style) in queries, as usual.
• Additionally, constructing BbcodeSettings now uses a builder pattern, which will scale better in the future. (breaking change)