tools/workflow: Add missing documentation for 'show'

`./tools/workflow s|show` shows a specific issue.
tools/workflow: Fail if a command doesn't execute
tools/workflow: Print usage if not command is given
tools/workflow: Push feature branches to remote

When:

  - creating a new issue branch
  - pausing work on an existing feature branch
  - closing a feature branch (delete the remote branch)
Update example 'workflow' usage in tools README.md

Now shows output incorporating ghi and gitflow.
tools/workflow: Refactor for features not issues

We're using branches for features, not for issues.
tools/workflow: Rename FEATURE_BRANCH_BASE

It wasn't an especially appropriate name to begin with.