Document the use of character fonts in the Howto

This will serve as our guide for actually implementing the feature.
Allow the ‘font’ property when constructing Character objects

This will teach the engine to accept and load fonts when creating
characters.  However, this will only affect the data stored in that
Character object.  The engine still will not use the character’s font
when rendering text.
Create TAGS entries for code implementing opcodes and instructions

This changes the call to ctags-exuberant so that it creates entries in
the TAGS file that will let us search for opcodes and instructions in
the code base, e.g. ‘Opcode.monologue’ and ‘Instruction.draw-image’
will jump to that opcode processor and instruction handler,
respectively.
Correct the comment for Scene:drawText() regarding the container

The source code comment claims that the method clears the scene
container on each invocation, erasing existing text on screen.  This
is not true anymore.  So we need to remove the out-dated comment.
Teach Scene:drawText() to optionally accept a font as an argument

We will use this to facilitate character-specific fonts.  All
character dialog goes through the following functions before ending up
on screen:

1. The ‘say’ opcode processor.
2. The ‘say’ instruction handler.
3. Scene:drawText()
4. Graphics.drawText()

The first two steps can handle an arbitrary amount of arguments,
whatever is part of the initial opcode arguments, while the last step
already requires a Font object.  We need to add support for passing
along a specific Font object to the third step in order to support
character-specific fonts, since then we will be able to pass them
along from the instruction handler.
Teach the ‘say’ instruction how to handle character fonts

This will make LNVL pass along any character-specific font data to the
Scene:drawText() method so that dialog by characters will use any
fonts assigned to those characters.
Merge branch 'character-specific-fonts'

* character-specific-fonts:
  Teach the ‘say’ instruction how to handle character fonts
  Teach Scene:drawText() to optionally accept a font as an argument
  Correct the comment for Scene:drawText() regarding the container
  Create TAGS entries for code implementing opcodes and instructions
  Allow the ‘font’ property when constructing Character objects
  Document the use of character fonts in the Howto