When passing values with "helm install --set" values with leading zeros are
preserved and not parsed as ints.

Closes #2693

This adds support for specifying list position with an array index using
`--set`. For example, this now works: `--set servers[0].port=8080`

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