Ignore trailing spaces in snippet definitions

Ignore trailing spaces in snippet definitions

Probably nobody wants a snippet `key` with a trailing space. It can
be hard to differentiate from a `key` without trailing space. The
default `<TAB>` behavior of yasnippet will never expand these.

* yasnippet.el (yas--parse-template): Ignore trailing spaces in snippet
  definitions.
* yasnippet-tests.el (deftest ignore-trailing-whitespace): New test for
  new behavior.

Copyright-paperwork-exempt: yes