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/commands/drawwhitespace.lua

@@ -0,0 +1,16 @@
+local command = require "core.command"
+local config = require "core.config"
+
+command.add(nil, {
+  ["draw-whitespace:toggle"]  = function()
+    config.draw_whitespace = not config.draw_whitespace
+  end,
+
+  ["draw-whitespace:disable"] = function()
+    config.draw_whitespace = false
+  end,
+
+  ["draw-whitespace:enable"]  = function()
+    config.draw_whitespace = true
+  end,
+})