Master files for pibook

The master files are coded in markdown, which has the following custom extensions:

• Figures and Epigraphs are stored in a YAML file and included with Image overloading.
• Notes are also stored in a YAML file and included with Note overloading.
• Extra files such Abstracts are stored in a markdown file and included with Image overloading.

The above extensions are easy to process during production with pandoc lua filters, or with cli tools such as bash, sed, awk.