Add custom templates

[behringer24]

Add a feature to make custom templates available.

The default stylesheet should only be added if the custome style/template has been selected.

Decide if this is an option on document level or should be available as command line option.

[behringer24]

Idea: Make the stylesheet cascadeable :-D

1. Load default stylesheet
2. Load document level stylesheet if set
3. Load compile time stylesheet if set
   This make the latter overwrite the previous ones.

To decide: Merge all to a single stylesheet OR add multiple stylesheets?
To check: Can e-book readers handle multiple files? Are there performance implications?

As the stylesheets are referenced at the top of each chapter file they must be set at the very beginning, before chapters are written.