Added more detailed documentation

For me bangsh balances simplicity and power in a great way. I wanted to learn how to use if and took some notes that I converted here to some more extensive documentation. Some things are pretty obvious, but the idea if to allow people to clearly see what bangsh can do for them. Let me know what you think!
It seems the `str` module is restricted to these 3 functions, maybe you should point to the API website http://bangsh.com/api/#String