Feature: Support opening MsDoc / Docx from buffer

[jneuff]

I have a use-case where I have document contents in a memory buffer. Using your crate, I need to write these contents into a temporary file to be able to open them as a Docx. Instead MsDoc could have a method from_reader (or something similar) that just takes a <R: Read + Seek> which would facilitate my use-case and similar ones.

I'll gladly open a PR for this feature! What do you think?