Improved efficiency (#1)

[studerluk]

Improve efficiency by not running pandoc twice.

This also includes following minor changes:

• Add recursive=True for globs
• Replace include type numbers with enum like variables
• Remove/replace utils/eprint with panflute pf.debug()
• Raise errors in case included files cannot be found

[DCsunset]

Thanks! The code looks more tidy and efficient after the refactoring.

[studerluk]

Happy to help!