Add support for the same variables as LaTeX document

[RLesur]

Some pandoc variables are implemented in LaTeX output format and not in HTML5 output format (see https://pandoc.org/MANUAL.html#variables-set-by-pandoc):

• institute
• abstract

This could be implemented through a customised pandoc template.

[RLesur]

Top-level variables in pdf_document:

• papersize
• fontsize
• geometry or margin-left, margin-right, margin-top, margin-bottom
• linestretch
• institute
• abstract
• fontfamily, fontfamilyoptions
• mainfont, sansfont, monofont, mathfont, CJKmainfont and mainfontoptions, sansfontoptions, monofontoptions, mathfontoptions, CJKoptions (relevant?)
• linkcolor, urlcolor, citecolor, toccolor (relevant?)
• links-as-notes
• indent
• thanks
• lof, lot