Added acrostic option

Hi. I've added an option to constrain the word choice to the first letters of a given word (i.e. "acrostic"). For example, 
`
./xkcd-password.py --wordfile 3esl.txt --acrostic FACE
floozy alcohol clueless e-mail
`

I tested it with both the interactive and valid-chars options, and it seems to play nicely. Care to integrate it?