Fix missing optional arguments to win over values

[svoop]

See #126 (comment)

Optional arguments (either implic or with explicit required: false) should be accepted when missing even if a values list does not include nil as a value – at least as per the principle of least surprise.

argument :required_arg, required: true, values: %w(one two)
argument :optional_arg, required: false, values: %w(one two)

cmd             # ERROR
cmd one         # OKAY
cmd three       # ERROR 
cmd one two     # OKAY
cmd one three   # ERROR

Not sure how best to spec this, hope it's okay to having added a vacuum subcommand to the db example.

/cc @gustavothecoder