Tag support on products (caffeine, alcohol, soda etc)

[jones-g]

As previously discussed it would be great if the Stregsystem v4 implemented a proper way of marking products for statistics, calculations etc.

One way of doing this would be to implement tags on products so that a cola soda could e.g. be tagged with "Caffeine" and "Soda" in the system. The same way for alcohol products but here also with a value indicating how much alcohol. The example being that a beer contains less alcohol than a bottle of schnaps.

This would allow doing things like calculating the blood alcohol level (albeit not exactly) and also make some statistics easier to calculate instead of the old way by having a list of product ids that have been staticly defined.

[fishbacon]

I'll take this one. Seems it should be part of #10