Add statistics

[frblo]

It would be nice for Rustwell to be able to calculate some statistics regarding the script, and output them (perhaps to stdout, or someplace else?). These statistics include:

• Number of scenes
• Number of characters
• Names of all characters
  • The number of lines they have
  • Number of words they say
• Which characters has lines in each scene

This output should be enabled with a flag, and the data should not be calculated otherwise. I think it would be good if we could do this at the same time that we export a document.