Add support for external config file
In order to allow configuration of mat2-web without having to alter the source (`main.py`), optional support for an external config file should be added. [With flask](http://exploreflask.com/en/latest/configuration.html#the-simple-case) it's as easy as this (after defining the defaults in `main.py`): ```py app.config.from_object('config') ```
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