As discussed in #25, we will probably eventually migrate to using the Properties service to make installation and configuration easier for non-technical users. These can be configured in the Apps Scripts UI shown in the below screenshot accessible from the File menu. I think it would be worth doing that regardless, so that we can all be running the same version of the code, rather than having to each maintain different configuration values.

This should be pretty straightforward – all we need to do is check for a Property (probably a User Property, as opposed to a Script or Document Property) for each configuration option, and then default to the current values if none is found.