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/config.lua

@@ -21,6 +21,7 @@ config.max_project_files = 2000
 config.transitions = true
 config.animation_rate = 1.0
 config.blink_period = 0.8
+config.draw_whitespace = false
 
 -- Disable plugin loading setting to false the config entry
 -- of the same name.