Document the arguments for every opcode
Rename LNVL.changeToScene() to LNVL.Scene.changeTo()

It makes more sense for that function to fall under the LNVL.Scene
namespace.  The Howto document already contains a couple of references
to changeTo() instead of changeToScene() anyways, so this patch also
makes the documentation and example scripts consistent with the code.
Rename Scene:draw() to Scene:drawEssentialElements()

The draw() method should be more generic and be the public-facing
method that programs use to render scenes instead of the method
drawCurrentContent().  The latter arguably exposes too much internal
information about the scene.  Programs uses LNVL should only have to
tell a scene to draw, not *what* to draw.
Re-introduce Scene:draw() as a public API method

The new version of the function is now the intended method for use in
code outside of LNVL, such as within the love.draw() function.
Update the main example to use draw() instead of drawCurrentContent()
Add two methods for moving forward and back through scene content

This will make it possible to abstract away the way we actually
navigate through scene content so that code outside of LNVL does not
have to deal with incrementing or decrementing the opcode index, or
whatever other approach we take in the future.
Use the new moveForward() and moveBack() in the main example
Merge branch 'abstract-scene-navigation'

* abstract-scene-navigation:
  Use the new moveForward() and moveBack() in the main example
  Add two methods for moving forward and back through scene content
  Update the main example to use draw() instead of drawCurrentContent()
  Re-introduce Scene:draw() as a public API method
  Rename Scene:draw() to Scene:drawEssentialElements()
Fix a small formatting mistake in the instruction documentation
Use title-case for all names in the LNVL.Color table

This helps improve consistency in the names since the ‘rgb.txt’ source
file sometimes uses title-case and sometimes lower-case for names.
Describe the title-case color names in the Howto document
Make it possible to convert LNVL.Color values to readable strings

This will help improve debugging by letting us more easily examine
color values on the console.
Merge branch 'color-title-case'

* color-title-case:
  Make it possible to convert LNVL.Color values to readable strings
  Describe the title-case color names in the Howto document
  Use title-case for all names in the LNVL.Color table