Added arrow options

[kenkomodo]

@pasahe While I was adding documentation, I noticed a bug with how the markdown text was not getting rendered properly. I fixed this in the DESCRIPTION file and then made some edits to the roxygen headers to maintain auto-links between packages (replacing the \link{} calls with simple square brackets that achieve the same thing). The thing that made me notice the bug was that the bullet points were not rendering in fc_export() details section the way they should have been before this fix.

After that was fixed, then I committed the updates to fc_draw() and fc_export() that were the primary concern for #43.

Example:

safo |>
  as_fc(label = "Patients assessed for eligibility") |>
  fc_filter(
    !is.na(group), 
    label = "Randomized", 
    show_exc = TRUE,
    perc_total = TRUE
  ) |>
  fc_draw(
    arrow_lwd = 3,
    arrow_lineend = "butt",
    arrow_linejoin = "mitre",
    arrow_color = "red"
  )

Result:

Closes #43

[kenkomodo]

@pasahe I rebased the commits for this PR onto the current main branch to preemptively resolve merge conflicts.

[pasahe]

It works beautifully! I will add the arrow_fill argument, in addition to arrow_color, to be able to change the color of the arrow head as well, and to be consistent with the options available inside lty_gp.