clean_class issue with numeric starting characters

[jameelmoses]

A valid CSS class may not begin with a number. https://stackoverflow.com/questions/448981/which-characters-are-valid-in-css-class-names-selectors#answer-449000

Steps to reproduce:

<div class="{{ '123 Clean This Class & Stuff'|clean_class }}"></div>

Expected result:

<div class="clean-this-class-stuff"></div>

Actual result:

<div class="123-clean-this-class-stuff"></div>

[RobLoach]

Thanks for catching this one!