No support for optional blocks

I'm writing a program where I have a large amount of config where the defaults are fine and don't need to be explicitly specified by the end user; I'd like to be able to do e.g. `hcl:"Policies,block,optional"` and have hclsimple be fine with that block not being present.

Start by locating hclsimple's decoding path and how its struct tags describe blocks. Add coverage for an optional Policies block that is absent while defaults remain valid, then run the relevant hclsimple tests to confirm both absent and present blocks behave correctly.

Written by the indexing model from the issue text.