Literate Jenkins

Initial implementation.

any indented text between the following single-line comments

```.. shell:: start```

and

```.. shell:: stop```

is run by scan.sh (which would be called by Jenkins). This allows for mixing of documentation and commands without needing to reformat entire documents.

Jenkins-only instructions would be communicated via a multi-line comment
```
.. shell:: special 
  echo only scan.sh sees
  echo this whole indented block
  
  echo yep, still in the block, not going to be processed by Sphinx
```
```.. shell:: special``` stills needs to be implemented. This might be better done as a Sphinx plugin.