Fix a bug where the Color module cannot open ‘src/rgb.txt’

We use the standard Lua io.open() function to read the list of colors
from disk.  This fails when we package the engine up as a demo inside
of a ‘*.love’ file because LÖVE places restrictions on how games can
access files.  It stops us from io.open() calls, even if they are in a
place that seems safe and innocent.  To fix this we need to replace
the io.open() call with the appropriate function from LÖVE’s
filesystem API.  That API adheres to the restrictions that LÖVE puts
on paths and will let the Color module correctly find files.
Make the command-line argument for the main demo script optional

This will let us more easily package a demo of the engine where the
user does not need to provide any script on the command-line.  This
will have no impact on LNVL itself though.  It is just for developer
convenience.
Use the LÖVE filesystem API inside LNVL.loadScript()

This will fix a bug where LNVL cannot scripts from inside of its own
‘*.love’ archive  because of the safety restrictions LÖVE places on
the standard Lua file-IO functions.
Rename Scene.foregroundColor to Scene.textColor

This name will more accurately reflect the purpose of that property
and make the naming consistent with Character.textColor, since the
latter property overrides the former when both are present and the
engine needs to render character dialog.
Provide an example of setting Scene.textColor in a script
Configure the default scene text color from the Settings module

This will allow developers and script authors to change the global
default text color for all Scene objects.  Individual scenes will be
able to override this by defining ‘textColor’ in their constructor,
and the text colors associated with Character objects will always
override both of those when a character speaks dialog in a scene.
Document changing scene text colors in the Howto
Merge branch 'scene-textcolor'

* scene-textcolor:
  Document changing scene text colors in the Howto
  Configure the default scene text color from the Settings module
  Provide an example of setting Scene.textColor in a script
  Rename Scene.foregroundColor to Scene.textColor