Support footnote for authors in NeurIPS template

[yuanxun-yx]

Issue: #58

Example:

#let authors = (
  (name: "Firstname1 Lastname1",
   affl: "skoltech",
   email: "author@example.org",
   footnote: [
    Use footnote for providing further information about author (webpage, 
    alternative address)---_not_ for acknowledging funding agencies.
   ],
   equal: true),
  (name: "Firstname2 Lastname2", affl: ("airi", "skoltech"), equal: true),
)