Add options to clobber or not clobber existing top-level sections



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My concern here is there are three ways of running the script (no argument - always import, `clobber` - replace existing, `no-clobber` - skip existing), but having options named like this implies it is a boolean argument (`clobber` or `no-clobber`). Could they perhaps be renamed to something like `--clobber-existing` and `--skip-existing`? The underlying behaviour could remain the same, though it might also be clearer to have two variables, e.g. `clobber` and `skip`. Note that optparse works on shortest matching variable name, so even if it's called `--clobber-existing` it can be called just as `--clobber`, so no change should be needed to your code afterwards.