take format string instead of only the number of words

[raspberryaether]

In addition to a number of words, we should also accept a format string which allows a password to be generated entirely programmatically if a user wishes to insert other characters between/after the words.

For example, the command: $ python -m dwpwg {@}-{+}-{.}-{.}-{#100}
Might output the password: CORRECT-Horse-battery-staple-42

Acceptance criteria:

• a format string scheme is implemented as described above
• README documents new scheme