Add Text field

[rossmacarthur]

It would be nice to have a field that can handle bytes and strings (strings and unicode in Py2). It could take an encoding value as well, otherwise defaulting to UTF-8.

Example:

from serde import Model, fields

class Example(Model):
    text = fields.Text(encoding='utf-8-sig')

assert Example(text=b'test') == Example(text=u'test')
assert Example(text=u'test') == Example(text=u'test')