--digits added, --specials no longer includes digits

[alanhoyle]

The changes implemented the following things to partially address issue #57

1. removes digit characters from SPECIAL_CHARS and therefore the --specials NUM output
2. adds a new --digits NUM option that inserts characters from the new DIGIT_CHARS variable
3. adds tests to confirm that both of these work.

It does not update the README.rst, to reflect these changes as I think that gets rendered somehow? I note that the developer documentation seems to be out of date as I had to jump through a couple hoops to get the tests enabled.

I had another idea that I might implement: modify the --delimiter option so that if it gets multiple chars, it selects one at random from the list. I.e. diceware --delimiter '1-#' -n 4 currently could output Correct1-#Horse1-#Battery1-#Staple but with that implemented, it would output: Correct-Horse1Battery#Staple