Add frontmatter

[oknozor]

• toml format :

transition = "some_cool_transition"
user_theme = "mytheme.css"
speaker_notes = " 
# Note to self 
Those notes shall be served on another port so 
the speaker can look at them while presenting.
We might also need a web socket interaction between the slide server
and the note server to synchronize slides & notes. 
"

• implement a preprocessor to separate frontmatter and markdown content

hint maybe take a look at zola implementation : https://github.com/getzola/zola/tree/1972e58823417a58eb1cc646ee346e7c3b04addb/components/front_matter/src