Add support for proc/lambda type checks.

Not sure if we want to read from metadata.rb, or from chef server API. This may end up in the companion cookbook.

verify value is one of a list of acceptable values.

When the value is nil, enum and type checks appear to be throwing violations, even if 'required' is not used. Investigate and determine documented behavior, then make sure that use case is covered.

Chef 12 is out, any plans to support it? Currently the gem is locked at 11.6.

When using attribute-validator cookbook, it is pulling in version 0.8.0 of chef-attribute-validator gem, which in turn is pulling in version 8.2.0 of the ohai gem, which in turn requires ruby 2.x.

Install fails with this error:

/opt/chef/embedded/bin/gem install chef-attribute-validator

Fetching: chef-attribute-validator-0.8.0.gem (100%)
Fetching: mime-types-2.4.3.gem (100%)
Fetching: mixlib-shellout-2.0.1.gem (100%)
Fetching: ohai-8.2.0.gem (100%)
ERROR: Error installing chef-attribute-validator:
ohai requires Ruby version >= 2.0.0.

We are using Chef 11.16, which is still on ruby 1.9.3

Possibly something is getting confused, thinking it is an array?