New crawl tags

Support for object, embed, and iframe html attributes added. Allows picking links out of those tag-types.
I'm trying this right now, so far I can't get it to work b/c the variables need to be byte arrays.

Variations on this

const (
	contentWordBytes  []byte = "content"
	dataWordBytes            = "data"
	nameWordBytes            = "name"
	noindexWordBytes         = "noindex"
	nofollowWordBytes        = "nofollow"
	robotsWordBytes          = "robots"
	srcWordBytes             = "src"
	srcdocWordBytes          = "srcdoc"
)

and this failed
const (
	contentWordBytes  = []byte("content")
	dataWordBytes     = []byte("data")
	nameWordBytes     = []byte("name")
	noindexWordBytes  = []byte("noindex")
	nofollowWordBytes = []byte("nofollow")
	robotsWordBytes   = []byte("robots")
	srcWordBytes      = []byte("src")
	srcdocWordBytes   = []byte("srcdoc")
)

compiler complains that the rhs isn't a constant. Also tried

const (
	contentWordBytes  = [7]byte("content")
	dataWordBytes     = [4]byte("data")
	nameWordBytes     = [4]byte("name")
	noindexWordBytes  = [7]byte("noindex")
	nofollowWordBytes = [8]byte("nofollow")
	robotsWordBytes   = [6]byte("robots")
	srcWordBytes      = [3]byte("src")
	srcdocWordBytes   = [6]byte("srcdoc")
)

Suggestions?