Support named task arguments

When creating tasks that take multiple optional arguments, specifying parameters becomes tedious.

When calling the following task

task :test, [:database, :table, :column, :host, :port, :user, :password] do
    # Something
end

and specifying only parameter user, I have to count its position and call rake as follows:

rake 'test[,,,,,testuser,]'

I suggest supporting a syntax using named parameters that makes calling these tasks easier and more readable, e.g.

rake 'test{user:testuser}'

I'd like to put this up for discussion, if it is acceptable, I'm willing to create a pull request.

The issue names no files, tests, or entry points. Start by locating task argument parsing and invocation handling, then determine whether named arguments such as rake 'test{user:testuser}' can be supported without breaking positional syntax; done means the proposed form works for optional parameters and is covered by tests.