Add support to docm files

[EmanueleCannizzaro]

Expected Behavior

It seems that a very small change to the main python file can allow the script to deal with macro enabled word file format.

Current Behavior

Today the .docm file are not usable.

Possible Solution

It consists of
...
CONTENT_TYPES_PARTS = (
'application/vnd.openxmlformats-officedocument.wordprocessingml.document.main+xml',
'application/vnd.ms-word.document.macroEnabled.main+xml',
'application/vnd.openxmlformats-officedocument.wordprocessingml.header+xml',
'application/vnd.openxmlformats-officedocument.wordprocessingml.footer+xml',
)
...

Initial tests suggest that change is sufficient to get most of the tests to process .docm files.

Steps to Reproduce (for bugs)

Context

I saved the projects docx files as .docm and tested the impact of the modification.

Your Environment

• Python version:
• docx-mailmerge version:
  latest git version