Custom dropdowns in block specs

I've been experimenting with the new scratch extensions and one of the things I would like to be able to do is create menus on blocks; instead of having a block for "temperature at zip code", "temperature at latitude", "forecast at zip code", "forecast at latitude", etc, one block with dropdown menus would be sufficient. This pull request modifies the block spec parser so that `%m.[option 1, option 2,...]` is understood as "dropdown menu with options option 1, option 2..." Not sure if there's interest but I would find it incredibly handy.