Add '.docm' compatibility

[1010G]

Hello, I saw few topics about '.docm' (docx with maccro) compatibility files so I decided to create a pull request. I think that could help some peoples.

api.py: modify
if document_part.content_type not in [CT.WML_DOCUMENT_MAIN, CT.WML_DOCUMENT_MACRO_ENABLED_MAIN]:

constants.py: append
WML_DOCUMENT_MACRO_ENABLED_MAIN = ( 'application/vnd.ms-word.document.macroEnabled.main+xml' )

init.py: append
PartFactory.part_type_for[CT.WML_DOCUMENT_MACRO_ENABLED_MAIN] = DocumentPart

[jshlbrd]

@scanny any chance we can get a review on this? Related to this 3yo issue #284

[lordofhyphens]

Looks pretty straightforward to me for minimal additional work.

[goncalor]

Hi. Will this PR not get merged? Thank you.

[scanny]

Not in its present state. There is no analysis documentation and no tests. The analysis document would need to account for how macros are stored in the .pptx package and document experiments that showed the macros were preserved on save and otherwise support reasoning about the mechanism used to establish support for .docm files and where it may be insufficient or have unintended consequences.

If this solution is good enough for your purposes you can just merge it into a private fork of the latest release and use that private fork.

[tsuga]

FYI, #716