This adds support for .helmignore files. These files roughly follow
the conventions established for .gitignore files:
https://git-scm.com/docs/gitignoreCloses#748
This provides support for "global" variables. It does this by
declaring "global" to be a special namespace. It then copies this
namespace into every subchart, coalescing it into any "global"
namespace found there.
The net result is that if "global.foo" is set in the YAML file, it
will be available to every chart/subchart as ".global.foo" regardless of
where that chart is in the subchart tree.
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.
