Add alignment_mode parameter to Span.char_span()

[itssimon]

Doc.char_span() has the alignment_mode parameter, but Span.char_span() does not. As a user I would expect both of these functions to be consistent.

[adrianeboyd]

No particular reason. I suspect users who wanted this probably just switched to Doc.char_span. Would you like to submit a PR?

[itssimon]

Sure, happy to give it a go!