regmap: Provide a map layer for maps of various regions
In cszang/eumap: Draw regional maps

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regmap(.data, region = "europe", .lon = "lon", .lat = "lat",
  .xlim = NULL, .ylim = NULL, .region_colour = "#BBBBBB",
  .region_fill = "#DDDDDD", plot = TRUE)

`.data`	data set to plot on the map
`.lon`	name of the longitude variable as character
`.lat`	name of the latitude variable as character
`.xlim`	longitudinal boundaries of the map
`.ylim`	latitudinal boundaries of the map
`.region_colour`	line colour for the region map
`.region_fill`	fill colour for the region map
`plot`	shall a plot be generated (TRUE, default) or only the map layer be returned (FALSE)