Fix parsing of escaped characters in attribute values

[hgoes]

The textString function discarded all text prior to an escaped
character. Example:
XmlParser.parse """<t attr="ABC&#253;DEF"/>""" => Ok { docType = Nothing, processingInstructions = [], root = Element "t" [{ name = "attr", value = "ýDEF" }] [] }

With the fix:

Ok { docType = Nothing, processingInstructions = [], root = Element "t" [{ name = "attr", value = "ABCýDEF" }] [] }

[miniBill]

This fixes #13

[dullbananas]

@jinjor please merge this

[miniBill]

(premise: thank you for writing this library, I appreciate that being an open source maintainer is a lot of work and people can have different priorities in life)

Do you expect you'll be able to merge this or should we fork the library?