Add validate subcommand functionality

[Cwagne17]

This PR integrates the CLI subcommand validate with the parse configuration functions.

Currently this is a draft to show what the implementation could look like. Just wanted to show what it looks like so far.

Some of the hold-ups:

1. Error handling, hcl.Diagnostics isn't appropriate for all the error types. We will want to switch over to use our own custom type.
2. Unsure what the plan for multiple configurations is
3. Unsure if we want to use a flag for validate or if just args[0] is ok.
4. Logs aren't the prettiest atm
5. Error handling at the end of the validate command should be abstracted