Proposal to set a proc when defining default values

[code-bunny]

When using Virtus.model we are able to set default values for attributes using procs which are currently missing from the properties hash.

Example usage:

property :id, default -> { SecureRandom.uuid }

Just now I have a after_initialize set that accomplishes this in more lines of code.

[senid231]

PR would be appreciated

[code-bunny]

I'll hop to it as soon as I can, it shouldn't be too tricky