Support --constant/-C <initial_constant_value> option

As discussed on the mailing-list, vw was taking very long to converge on "non-centered" data-sets where the constant is far from zero.

This change adds an option (either --constant <arg>, or -C <arg>) which allows setting the constant to an initial value.

A test-case was added to show how good the convergence is on a data-set whose label is centered around 1000 (assuming this fact is known in advance), when -C 1000 is given on the command-line.