range-faces are now used to replace-range highlighters, where the string
part is not interpretted as a face but as a display line, so the name was
not relevant anymore.
This commit adds the following flags to the `show_whitespaces`
highlighter, with a one character long parameter:
* `-lf`: character replacing line feeds
* `-spc`: character replacing spaces
* `-nbsp`: character replacing non breakable spaces
* `-tab`: character replacing a tabulation
* `-tabpad`: character used as padding after a tabulation to satisfy
the `tabstop` option
