Feat: Allow a user to pass their own Markdown

[esacteksab]

Our PR body is very sparse. We don't provide a mechanism for the user to edit the body prior to creating the pull request.

Though a workaround for that does exist today and that would be to simply use gh natively.

gh pr create -t 'my title' -F myfile

Ideas:

• the most direct gh tp -m file.md
  • This would bypass creating the plan, but redundant to the gh command above
  • -m could append to the file we create with gh tp
  • -m could take stdIn allowing a person to author additional text that could be prepended or appended to the plan file
  • Could dump to $GIT_EDITOR or $EDITOR with the body of the markdown and allow them to take it from there. Saving/Writing the file would take that file and use it as the body of the PR.
• Custom Templates
• Additional flags to be passed that gather environmental information (file(s), director[y|ies], commit messages) and append them to the Markdown's body prior to creating the PR.