Added front-matter parameter to #algo().

[nervecenter]

Added front-matter parameter to #algo() which places given content below the header, but above the algorithm table and line numbers.

Example:

#algo(
  title: "Fib",
  parameters: ("n",),
  front-matter: [
    *Input*: The number in the Fibonacci sequence $n$ to generate.\
    *Output*: The $n$th number in the Fibonacci sequence.
  ]
)[
  if $n < 0$:#i\
    return null#d\
  if $n = 0$ or $n = 1$:#i\
    return $n$#d\
  \
  let $x <- 0$\
  let $y <- 1$\
  for $i <- 2$ to $n-1$:#i\
    let $z <- x+y$\
    $x <- y$\
    $y <- z$#d\
    \
  return $x+y$
]

Outputs the following: