Detect and offer to add nested .gitignores to .bentoignore

[dlukeomalley]

Observed Issue

Nested .gitignores need to be manually added to .bentoignore for Bento's to be performant. This tripped up me and a user, and was resolved by the above. This seems like an easy mistake to make and is an opportunity to substantially improve the initial experience (can take runs from minutes down to seconds).

Possible Solution

During bento init, prompt the user if nested .gitignores should be appended to .bentoignore.

[dlukeomalley]

We plan to address #209 before this issue is considered.