Tokenize tags and mapping keys with proper order

[kpodsiad]

Similar to the #87.

See related testcase:

scala-yaml/yaml/shared/src/test/scala/org/virtuslab/yaml/tokenizer/TagSuite.scala

Lines 75 to 94 in 5b97381

	// todo Tag token should be placed after MappingKey
	test("verbatim tag".ignore) {
	val yaml = """|---
	|!<tag:yaml.org,2002:str> foo :
	| !<!bar> baz""".stripMargin
	val tokens = List(
	DocumentStart,
	MappingStart,
	MappingKey,
	Tag(TagValue.Verbatim("!<tag:yaml.org,2002:str>")),
	Scalar("foo"),
	MappingValue,
	Tag(TagValue.Verbatim("!<!bar>")),
	Scalar("baz"),
	BlockEnd
	)
	assertTokenEquals(yaml.tokens, tokens)
	}