Add a minimal bob-walker in brainf--k

This is an MVP of bob-walker implemented in the ever popular [brainf--k](http://en.wikipedia.org/wiki/Brainfuck) programming language.

- It accepts no arguments - you have to pass in a mode on stdin
- It's not capable of running via shebang on most systems (I've suggested an external interpreter, as per the awk version)
- It runs through wordlist 10 times rather than infinitely, because you brain can only be f--ked so much.

Patches as always are welcomed with open arms.

You may find [this](https://github.com/garretraziel/mindfuck) quite helpful.

```
# Normal mode
$ echo "0" | ../mindfuck/mindfuck.py bob-walker.fk
beer
beard
pie
...

# Sport mode!
$ echo "1" | ../mindfuck/mindfuck.py bob-walker.fk
beer
beard
pie
cricket
rugby
...
```

I have generously supplied [a commented version in the docs folder](https://github.com/dcarley/bob-walker/blob/741b73cf88bb65cc86d6823a84343a0df360f69e/docs/bob-walker-explained.fk) just in case your brain has been f--ked too much already today.