Add 'mount' variable to Entries - Take #2

[duncanmcclean]

This PR implements a new mount variable to entries when they are augmented, as suggested by Jason on my previous PR.

This feels a lot cleaner than doing it directly on the Cascade class. Let me know if there's any changes you'd like me to make to this.

This PR implements #1352.

[duncanmcclean]

I've converted this pull request to a draft until I get round to fixing the tests.

[jasonvarga]

I adjusted it so that if someone manually set a mount variable, that would take precedence. For instance, if someone wanted to recreate the v2 behavior, they might have added their own mount variable to the entry.

Otherwise, nice one.