Add increment-clojure-cookbook.el.

If you're reading this book in Emacs, this function finds the next
recipe.
Punctuation in README.
Move increment-clojure-cookbook to README.

Also, add documentation and key binding.
Use Github-friendly language tag for Elisp.
Cleaner implementation.

This is more lines, but I think it's easier to read.
Fix sub-dir / next-dir bugs.

This implementation has two bugs: if the next recipe is located in a
sub-directory, or if the chapter has ended. Rather than hard-coding a
solution, this fixes the problem by closing the buffer and opening the
directory in Dired. Not ideal, but it's too hairy to do otherwise.
Steal and document Ryan's CIDER hook.
Document sub-dir / next-dir bug solution.
Merge pull request #426 from mwfogleman/increment-clojure-cookbook.el

Add increment-clojure-cookbook.el.