Allow dash character in tag selector names.

This all started while trying a test from upstream LESS project.

In the upstream test 'pi-test' was used as a selector.

I think that lesscpy should support this.

Here is an article about custom elements:

http://www.html5rocks.com/en/tutorials/webcomponents/customelements/

This branch is based on my previous work, so until the parent branch are merged
this can not be merged/reviewed.

Thanks!