RFC: add in()

Hi,

I made a very quick addition to the methods: `in(a,b)`, which basically calls `contains(b,a)`. This allows
```
julia> "Hello" in bf
true
```
which is nice for quick checking. I updated tests and the main module as well to support this new method (just put it in one loop to make sure it works).

A small change, but I hope it's useful. Thanks for this package!