Add Django setting to configure jsx flags

[frankier]

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[kmeht]

Thanks for the contribution! Would you please add a small test and documentation in the README?

[andreypopp]

Does this add a dependency on Django?

[kmeht]

django-pipeline depends on django, so it wouldn't be adding a dependency to a user's project. But I suppose it does add the dependency here if we want to test it.

Alternatively, we could catch the ImportError and use an empty dictionary?