Fix double separator showing in snippet preview

Last bit from issue #1321
Stricter custom replacement variable names

* Allow uppercase letters and dash in custom replace variables (js)
* Restrict custom variables name to [A-Za-z0-9_-] characters
Fix issue 1333 - slashes in snippet preview
Fix issue #1343 - re-introduction of bug blocking Soliloquy
whitespace *sigh*
Updated changelog
Merge branch 'Js-fixes-x-3' of https://github.com/jrfnl/wordpress-seo into jrfnl-Js-fixes-x-3

Conflicts:
	readme.txt
Merge branch 'jrfnl-Js-fixes-x-3'