Add --header command line argument

[ghost]

The argument allows users to specify one or more additional headers which will be set for all requests.

The use case which prompted this change is a confluence instance behind an authentication proxy. Instead of adding support for individual types of proxies, I opted to add a generic --header option.

Usage example:

python markdown-to-confluence.py --header 'Cookie: X-AUTH-TOKEN=123' test.md

or

export CONFLUENCE_HEADER_Cookie='X-AUTH-TOKEN=123'
python markdown-to-confluence.py test.md

[jordan-wright]

LGTM - thanks @timatduo!