Support ::first-letter pseudo element

[frewsxcv]

Currently, we just return an error:

rust-selectors/src/parser.rs

Line 578 in a6cf1fb

"first-letter" => return Err(())

http://dev.w3.org/csswg/selectors-3/#first-letter

[SimonSapin]

Same comment as #40 (comment)

[SimonSapin]

Pseudo-element parsing and matching is now generic in this repository, moving to servo/servo#15413.