Allow DocXParser to take a filepointer instead of a string path

Newer versions of pydocx broke backwards compatibility by requiring that a path be passed as an argument to `pydocx.DocXParser.DocXParser` instead of a file-like object. 

I see this as a regression in the API because there are use cases where the client will not be able to give a full path. 

Would it be possible to allow a file-like object to be passed to the DocXParser? The COS's internal modular-file-renderer project depends on this functionality, so we are currently pinning to version 0.3.14.
Hooray documentation!