Escaping.validAttrName disallows a leading understore

[malcolmredheron]

The comment on Escaping.validAttrName says that an attribute can start with an underscore but startCharValid doesn't allow an underscore. I think that the fix is as simple as || sc == "_" but I'm submitting this as an issue rather than a PR because I haven't taken the time to make a fork, edit, build, run tests, etc. Sorry!

scalatags/scalatags/src/scalatags/Escaping.scala

Line 27 in f087694

val startCharValid = (sc >= 'a' && sc <='z') || (sc >= 'A' && sc <='Z') || sc ==':'