support pseudo-elements in pesticide?

[nchase]

this is a naive changeset in the interest of support for pseudo-elements in
pesticide. it's not ready to be pulled in, but I wanted to get a little
bit of discussion going. here are the notes:

1. this changeset only adds outline support. to fully support
   pseudoelements, you'd probably want to add depth rules for before and
   after too.
2. didn't mess with color_table.json. should there be entries there
   for before and after too?
3. I manually added rulesets for stylus, less, and scss. this is a
   fairly-complete, awesome library, but what's the best process to follow
   when a change does need to be introduced? (i.e., of the steps I've taken
   here, what's missing?)
4. looks like the current css/pesticide.css is actually a stylus
   file. is that just a bug? in my changeset here, I wrote the file as
   though it was a stylus file to avoid formatting differences.
5. I didn't re-generate the minified CSS file. regenerating that file
   from master looks like it's significantly different from whatever's
   checked in, so I didn't want to add any noise. probably best to handle
   whatever's going on there in another branch, assuming there's an issue
   there.

[juni-vogt]

Pseudo elements are relevant for layout bugs, they should be outlined too.