Fix indentation for the comment for Scene.activeCharacters

This patch contains no changes to logic.  It only fixes the
indentation of the first line of a comment block so that everything is
properly aligned.
README: Add Yusuke Tanaka to the ’Special Thanks’

He deserves mention for making LNVL compatible with the latest version
of the LÖVE engine.
Add comments explaining why the LNVL table cannot be ’local’
Improve error output in LNVL.LoadScript()

This patch adds a message to the assert() that checks the metatable of
LNVL.CurrentScene to make sure it is an LNVL.Scene object, so that if
the assertion fails we can see the reason why.
Correct two bugs with the example for scene preconditions

1. The script does not define any scenes because it does not call the
Scene constructor, causing an assert() to fail in LNVL.LoadScript().

2. The precondition scene list for ’BATHROOM’ has an accidental and
incorrect space at the end of the scene name.
Characters: Accept color names for text and border colors

When creating a character in a dialog script the Character constructor
will treat strings for properties like ‘textColor’ as hex color
strings, e.g. ’#33cfaa’.  This fails to account for situations where
the string explicitly names a color, e.g

    Eric = Character { textColor = "Black", ... }

This patch fixes this bug by first testing if a given color string is
a valid key in the LNVL.Color table.  If so then we use the table of
RGBA values mapped to that name.  Failing that we revert to the
original behavior of treating the string as a hex color value.
Remove debugging for-loops from the ‘set-scene’ instruction
Correct the whitespace for the ‘set-scene’ instruction

This patch contains no logic changes.  It only corrects the
indentation for the instruction so that it is consistent.
Fix two typos in a comment for the ‘set-scene’ instruction
Do not quote the number of opcodes in Scene.__tostring()

This is a slight and trivial improvement to debugging output.
Add a debugging function to dump the script environment

This gives us a way to see all of the variables in the
LNVL.ScriptEnvironment table along with their types.  This does not
give us specific type output when the values are tables, e.g. we do
not see whether a table represents a Scene object versus a Character.
However, this basic amount of output lets us quickly see what
variables exist in the environment, which is useful for debugging.
main: Dump the script environment after loading scripts

This modifies love.load() so that we dump the script environment if we
are running in debugging mode.  The intent is to help us track down
current bugs in example scripts that change scenes, as they are
crashing with errors about not being able to find scenes in the script
environment.