Add first and last class functionality

 * Adds 'charfirst', 'wordfirst', 'linefirst' to the first character,
   word, or line, respectively.
 * Adds 'charlast', 'wordlast', 'linelast' to the last character,
   word, or line, respectively.
 * Optimizes injector method to use array joining instead of string
   concatenation
 * Completes https://github.com/davatron5000/Lettering.js/issues/19