New dsl method: lazy(&block)

[lionelperrin]

Make possible to lazy define tasks in a namespace. The tasks are
loaded only on lookup.

Lazy definitions can be executed on purpose when using the option
--load-lazy-definitions so that rake --tasks --load-lazy-definitions
list all tasks including the one defined in a lazy way.

Example of lazy definitions:

namespace 'a' do
  lazy { task 't' }
end

or

namespace 'a' do
  lazy { load 'namespace_a.rake' }
end

[TSMMark]

This is awesome! I am in need of something like this, and would love to see this merged and released if possible.

[TSMMark]

I'm surprised this doesn't have more traction... Everyone enjoys longer load times because they're loading extra code they don't need all the time I suppose?

Any words from maintainers what it would take to get this merged? Thanks!