Add a minimal bob-walker in brainf--k

This is an MVP of bob-walker implemented in the ever popular brainf--k
programming language.

- It accepts no arguments
- It runs only once through the loop

Patches as always are welcomed...

You may find [this](https://github.com/garretraziel/mindfuck) quite
helpful.

```
$ ../mindfuck/mindfuck.py bob-walker.fk
beer
beard
pie
```