fix: Token.CharData should check underlying event type (#14)

Checking the type of the token compiles and partially works because both event types and token types share a type and gosax.EventText == xmllb.CharData.

fix: add syntax error

fix #13

fix: panic on invalid attribute

close #12

fix: fix a bug in Skip

feat: add xmlb.Decoder.Text

docs: update README

docs(xmlb): add package comment