`output_file` setting should be `changelog` instead

[nedbat]

Originally, scriv only wrote to changelog files, so output_file seemed right. But the github-release command reads from the file. A better name would be changelog (or changelog_file) for the setting. Of course, the old setting would still be recognized so people wouldn't be broken.

[nedbat]

Also, insert_marker should be start_marker, especially since the corresponding setting for the end is already called end_marker.