Allow cli version to run from texproj or _partial_ texproj files

[NeilMacMullen]

It would be quite neat if the UI could be used to put together a template and possibly definitions and helper models and then this could be fed to the cli tool as a single control file. So the flow might be ....

• Develop models and template in UI
• Export version of proj without "main" model"
• use cli tool to run proj with new model

This would be particularly neat for text-processing applications where the user could build a number of standard "recipes"

Related to #20