Implement unicode character replacements

Useful to draw whitespaces with alternate characters and colors
without slowing down the text rendering.

A new API is implemented. A renderer.replacements object can be created
to list the replacements.

In turns the function renderer.draw_text and draw_text_subpixel now accept
two optional arguments for replacements.

data/core/command.lua

@@ -59,7 +59,7 @@ end
 
 
 function command.add_defaults()
-  local reg = { "core", "root", "command", "doc", "findreplace", "files" }
+  local reg = { "core", "root", "command", "doc", "findreplace", "files", "drawwhitespace" }
   for _, name in ipairs(reg) do
     require("core.commands." .. name)
   end