Add support for DBIx::Class "nested" Changes files

I realize this is a long shot getting this merged, but I figured it would be worth giving it a shot :smile: 

See CPAN-API/metacpan-web#897 for the bug that triggered this experiment

This might not be "in line" with the spec, but it makes metacpan-web look nicer
at least :)

The "nested" support is quite hackish, and I haven't tested deeper nestings
than what DBIx::Class uses.

Another option (and maybe less "invasive") could be to have an option to
preserve newlines in the changes entries, which would allow metacpan to format
it as a `<pre>` block perhaps..