Deal with basic subset of LaTex to extend markdown about franklin.jl

[tlienart]

Basic solution to capture definition of commands in a0df2ac note

• now have a Dict with \\comname => (narg => def)

Todo

• would be better to have a latex type instead of a pair, and form a function out of it that would directly be the appropriate replacement + also the search regex...
• maybe we could allow dropping the [ 5 ] and just retrieve the number by the # but bug if the order of # is mishandled or something

[tlienart]

now MWP