Add back DDL mode

[matthewmturner]

The original version of dft had a "DDL" mode that made it very quick and easy to load your DDL file into the editor and then save it back to disk. At the time I found a lot of value in this and I think we should add it back.

It would also lay the groundwork for 2 other modes I have in mind - "Analyze" (Collect plan / node execution metrics after execution) and "Benchmark" (automatically run the selected query X number of times and generate some summary statistics / visualization based on that.