Very basic implementation of interactive mode
Introduce 'exec' route; refactor
Merge branch 'master' into 75-interactive
Rename system testing script
cut operator unit tests.

Refs #95.
Remove most of the dependence on row index
Make the second arg to replace optional.

Defaults to empty string. Refs #77.
Merge branch 'master' into 75-interactive
Catch errors rather than crashing.
When something breaks, send a message to the user
Strict mode! processing halts on non-fatal errors
Updated interactive mode.

Doesn’t play nicely with the html operator yet.
Merge pull request #98 from andylolz/97-handle-errs

Catch errors rather than crashing.
Add a loading spinner in interactive mode
Interactive now works for html output
Move class to body element
Merge pull request #99 from okfn/75-interactive

[#87]: working interactive mode - #87.