Add ConfigResolveOptions.allowUnresolved

This allows Config.resolve() to do a partial resolve.

Fixes #100
add a Config.isResolved method

Now that partial resolution is allowed, this may be useful
for people to check after they do a partial resolve.
add Config.resolveWith()

This allows replacing substitutions with values which
are not merged into the Config.

Fixes #95
Merge pull request #124 from typesafehub/wip/havocp-fine-resolve-control

Add better control over the resolution process