Support for '.docm' OOXML format? #212

Hello,

It seems that python-docx does not have any built in support for '.docm' OOXML documents. From the perspective of parsing this document type as a docx, there is not much difference: There are some extra document parts that support the macro language / VBA, and the Content Type for the 'word\document.xml' Document Part is 'application/vnd.ms-word.document.macroEnabled.main+xml'.

Attempting to open a '.docm' with python-docx results in the error code "file '...' is not a Word file, content type is 'application/vnd.ms-word.document.macroEnabled.main+xml'", and it does not even attempt to parse the file.

However, because a docm is basically a docx, I can get python-docx reading and writing docm files by adding the 'application/vnd.ms-word.document.macroEnabled.main+xml' Content Type to 'opc/constants.py', assigning the Content Type to a Part Factory in init.py, and adding the new Content Type to the gatekeeper check in api.py.

Is this a feature you might add to the library? So far, it seems like a small change; I haven't had any issues with my locally patched version yet, though you might know of some gotchas that I haven't encountered yet.

Thank you,

Pete