This adds support for .helmignore files. These files roughly follow
the conventions established for .gitignore files:
https://git-scm.com/docs/gitignoreCloses#748
This is a refactor of the loader to use in-memory buffers instead of
trying to optimize for memory usage by delaying reads until the last
possible moment. Since charts tend to stay well below 1M in size, this
makes more sense, and is easier to work with.