--project=@. for Julia and support for flags based on version

[gpoore]

Julia support may need --project=@. or possibly a way to add custom --project. This only exists for Julia >=v0.7.

This raises the question of whether there should be a system for adding user-supplied flags to the run_command template, or allowing the user to override run_command completely. It also suggests that version-conditional run_command might be useful in some cases (even if it is ultimately avoided for Julia by dropping support for <v0.7).

Related PythonTeX issue: gpoore/pythontex#158