load MarkupText from XML string

If I want to correct a title using the library, is there a simpler way than:

```py
from lxml.etree import fromstring
vol.title = MarkupText.from_(fromstring('Proceedings of the ACL 2001 Workshop on Computational Natural Language Learning (CoNLL)'))
```

I.e., first creating an XML element? I had tried `from_xml()`, but that escapes all the XML metacharacters. If that's intentional, something like `from_xml_str()` would be nice to have.