Handle escape sequence in extenstion fields

[masatake]

Though escape characters like \t in names are unescaped well, those characters in extension fields are not.

An example:

level2	input.rst	/^level2$/;"	s	scope:chapter:\x01level1

readtags stores "\x01" as is to the value field of tagExtensionField type.
It should not do. Unescaping is needed here.