This is another useful package outside of cmd/helm.

This replaces the old set parser with a brand new one. This also changes
the internal algorithm from duplicating YAML to merging YAML, which
might solve a problem one user reported in chat, but which was never
captured in an issue.

Closes #1540
Closes #1556

This also refactors significant portions of the CLI, moving much of the
shared code into a library.

Also in this release, a testing repository server has been added.