Add man page tempalte.

We should be able to generate a more accurate man page for `diceware`.
with `docutils` and `rst2man` we can create a nice looking man page with
proper styles set.

Yet, we onlyy provide first parts of the man page template.

docs/manpage.rst

+:title: diceware
+:title_upper: DICEWARE
+:subtitle: create passphrases
+:manual_section: 1
+:manual_group: User Commands
+:date: August 2017
+:version: diceware 0.9.2.dev0
+:author: Written by Uli Fouquet
+
+.. :authors: Uli Fouquet and Contributors
+
+
+
+synopsis
+--------
+
+``diceware`` [`OPTION`]... [`FILE`]...
+
+
+description
+-----------
+
+options
+-------
+
+``positional arguments``:
+
+  INFILE
+    input wordlist. ``'-'`` will read from stdin.
+
+``optional arguments``:
+
+  -h, --help
+    show help message and exit
+
+  -n NUM, --num NUM
+    number of words to concatenate. Default 6
+
+copyright
+---------
+
+Copyright (C) 2015-2017 Uli Fouquet and contributors. License GPLv3+: GNU GPL
+version 3 or later <https://gnu.org/licensesi/gpl.html>.
+
+This is free software: you are free to change and redisttribute it.  There is
+NO WARRANTY, to extent permitted by law.
+