Provide alpha levels for maps

[MichaelChirico]

It's pretty easy to adjust the alpha level of the map if it's being used as a background, e.g.

alpha_to_hex = function(pct) sprintf('%02X', round(255*pct))

# bg_map: an OpenStreetMap object from openmap()
alpha = .5
if (alpha < 1) {
  alpha_hex = alpha_to_hex(alpha)
  for (tile_i in seq_along(bg_map$tiles))
    bg_map$tiles[[tile_i]]$colorData =
      paste0(bg_map$tiles[[tile_i]]$colorData, alpha_hex)
}

I'm not familiar enough with the Java interface to know if this can work at a lower level, but this seems to work pretty well from what I've been playing with thus far.

I'll file a PR if this sounds useful.

[ifellows]

sounds like it could be useful. Feel free to add a pull request