enable external editors

[jimy-byerley]

When the user desires, it should be able to use an external editor, and uimadcad should automatically reload and reexecute the file when it is modified. rexececution should follow the same incremetal optimizations as described in #22

Such user won't be able to benefit interactions between their external editor and the uimadcad 3d view, but selecting objects in the 3d view or running into errors should still highlight them the matching code in the uimadcad text view