Make default snippet customizable
Factor out embedded elisp code

* yasnippet.el (yas--key-from-desc): Added
Assume NO-TEMPLATE is non-nil if `yas-new-snippet-default' is nil
Merge pull request #397 from thisirs/customizable-snippet

Make default snippet customizable
fix: more idiomatic elisp, better docstrings, less byte-compiler warnings

By Stefan Monnier, with original commentary.

"Here's an untested patch of cleanups, found partly via compilation using
lexical-binding and partly via checkdoc-current-buffer (and
occasionally by stumble-upon happenstance)."