Implement init_arg for attributes

[schwern]

init_arg is the expected key in an initialization hash.

For instance, if we have an "init_arg" value of "-foo", then the following code will Just Work.

MyClass->new( -foo => 'Hello There' );

• init_arg defaults to the attribute name
• If init_arg is explicitly set to undef the attribute cannot be specified during initialization.

See Class::MOP::Attribute for the original reference.

[tobyink]

I've moved the Mite issue tracker here.

This is now implemented.