Add HTML head meta icon customization

[JousP]

It should be nice to add a directive to set the favicon for html slidedecks.

[yhatt]

🚚 Transfered the issue from Marp Core to Marp CLI. Marp Core only outputs the snippet of the slides. Thus, the core cannot output the definition of the favicon for the whole of HTML.

Related: marp-team/marp#242

It may be provided as part of the metadata directives provided by Marp CLI.

However, there are already many directives in Marp, and adding more directives may overwhelm beginners, so the Marp team is hesitant to implement that. Pull reqeusts from community is welcome.