Remove bash prompt from exec blocks

[jonocodes]

If a line in an input block starts with '$ ', then '$ ' would get removed before executing it. This way the markdown can follow the Docker/Arch documentation guidelines a little closer. https://docs.docker.com/opensource/doc-style/#code-and-ui-text-styling

This could also be done with root commands which conventionally start with '# '.

[jonocodes]

This has been mentioned upstream in the PWD.js project. Follow it here play-with-docker/sdk#11

If it is not done there, perhaps pwder can implement it in Javascript.