Strings Cookbook

For your convenience, you can [read it here](https://github.com/tjg/guides/blob/strings-cookbook/articles/cookbooks/strings.md).

I hope this covers the most common string patterns. I'm happy to receive feedback and edit this cookbook further (even nitpicky micro-edits are welcome).

Potential concerns some may have:

* The basic recipes section is a long code block. The justification is that it improves skimmability.
* Instaparse is covered, even though it's a 3rd party lib. If I cover regexes, I think context-free grammars are also natural to cover.
* `cl-format` is covered. At least I provide many warnings. And it is included in Clojure itself, so might as well learn to defend yourself from people like me. ;)