Feature: Implementation of RFC Attributes

[chris-rock]

This PR implements parts of RFC Attributes #719

To use it:
$ inspec exec examples/profile-attribute --attrs examples/profile-attribute.yml

An attributes file looks like:

cat examples/profile-attribute.yml 
user: bob
password: secret

The usage within a profile is as following:

val_user = attribute('user', required: true)

describe val_user do
  it { should eq 'johndoe' }
end

[arlimus]

Inverting feels more readable:

describe val_user do
  it { should eq 'bob' }

[arlimus]

ditto vv

[chris-rock]

Yeah, I like that...

[arlimus]

Awesome work @chris-rock !!

Adding the ncessary tests should round this up.