Suggestion: Allow certain fields to be optional: true

[Qqwy]

The idea is as follows:

• When a field has no default set, it is marked as required and Specify will raise when nothing ends up being passed.
• When a field has a default set, this is what Specify will fall back to.
• When optional: true is set (and there is no default set), Specify will accept the value nil on top of what the field parser accepts, and will use this as fallback-value.

I encountered the usefulness of this while writing Blocked.Config where most of the fields are optional. Since Specify currently does not accept nil as default when the parser e.g. is string or boolean, this becomes a problem.