Allow setting heading indentation level

[stefanv]

In some changelogs, you have multiple versions listed together. There, I want the initial entry to start with ## and other headings to be lower than that.

[lagru]

Hmm, I wonder how this would interact with the restructuredText format. Since restructuredText is ordering depth in order of different underline characters as they are encountered, this should already be supported for rsT.

I guess this should be a command line option instead of a config option?