Add Font#text_word,#text_grids , to assist with font creation.

Add `Font#text_word` to join output from `#text`, suitable for display
on screen. This is a basic implementation, which could well be improved
by refactoring across `Font` (lines are joined with `\n` and then split
on `\n` again). This provides swift feedback during the preparation of
fonts. e.g.

    ruby -e "require './font'; f = Font.new('./fonts/default.yml'); puts f.text_word('peace')"

Add `Font#text_grids`, for displaying entire font on screen. This
iterates through all grids defined for a font, displaying as many
grids per line using `#text_word` as fits within `word_width`, adding
spaces between lines. This enables an entire font to be output to screen.