Change colour scheme selection, add viridis colour map.

* include/viridis_lut.h: New file.
* include/colour.h (colour_type): New enum.
(colour_function_t): New typedef.
(get_colour_mono): Add `colour_type' parameter.
(get_colour_cividis): Rename to ...
(get_colour_cmap): ... this and add `colour_type' parameter.
* src/colour.c (get_colour_mono): Update parameters here.
(get_colour_cividis): Rename to ...
(get_colour_cmap): ... this and update parameters.
* src/main.c (init_options): Switch `monochromo' parameter to
`type' to indicate colour.
(fill): Use `colour_function_t' typedef.
(main): Switch from option `-m' to `-t <type>' to select colour scheme.