Wordlists path is hardcoded in API doc

[drebs]

The path to wordlist files is hardcoded in the generated sphinx
documentation and because it may differ depending on how the package is
installed this creates different problems:

• the path hardcoded in the online api documentation may differ from
  the one in each installation, leading to incorrect information
  provided to the user of the api.

• packaging diceware documentation to specific operating systems may
  generate unreproducible packages.

One possible solution is to replace the WORDLISTS_DIR with a function called get_wordlists_dir() and have the path be constructed during execution of the function rather than during module load time.

[drebs]

This is a proposal of fix: #38

[ulif]

That's great, thank you @drebs !
Just merged your code. I hope this will help with reproducibility :)