Allow colons and slashes in attribute shortcuts

[tensiondriven]

https://github.com/slime-lang/slime picked up support for Tailwind CSS recently by supporting colons and slashes in class names, see:

slime-lang/slime#163

Phoenix slime would do well to pick this up as well.

For reference, here's the commit against slim that adds such support:

slim-template/slim@b5e2cda

[silverdr]

FWIW I recall using both colons and slashes w/o problems for quite some time. So might it be that the "upstream" change in slime-lang/slime was all that was needed?