Option to insert command output back into markdown

[benkrikler]

Extend the testing tool by having it run a markdown file and insert each command blocks output into an output block. If one exists, it should be replaced, if nothing exists it should be created (or perhaps optionally ommitted, if that's what the user intended originally). Makes the testing tool even more like an org-mode / notebook tool for markdown and bash, and useful as a way to update docs automatically when a command has been changed.