Country harmonizer improvements

[spoonerf]

I had a couple of ideas for small improvements to the country harmonize:

• Add an 'exclude' option (similar to skip and custom) to the harmonizer CLI, which would automatically add the entity to excluded_countries.json
  

• We shouldn't automatically assign entities that happen to have the same name as OWID continents to those entities, e.g., Africa in a new dataset currently automatically becomes OWID Africa, and it may actually be defined differently. Not a huge deal but it is an extra step to remember.