Allow shipping of child branches into their parent

[kevgo]

Several developers have described scenarios where shipping a feature branch into its parent feature branch makes sense:

1. Building a larger component out of smaller feature branches, and then only shipping the larger system all at once when it is built complete. Clarification on child feature workflow #1178 (comment)
2. While developing a tricky algorithm in a feature branch, try out a number of alternative implementations in child-branches, then merging the winner into the feature branch and ship it. Clarification on child feature workflow #1178 (comment)

Git Town should encourage best practices but allow deviations from them. Let's add a --to-parent aka -p flag to git ship that allows shipping into a non-perennial parent branch. #1178 (comment)