Refactored md2man/rakefile into a Tasks class

Converted the `md2man/rakefile` into `Md2Man::Tasks`:

* Accepts an `:options` option for passing options down to Redcarpet.
* Accepts a `:web` option for enabling/disabling generation of `.html` files.
* Aliases the `build` task to `md2man`.

In the future more options could be added to customize the format of the `.html` files.