Add a command for generating a legal boilerplate page

As @hvr mentioned in #4883, a lot of libraries `cabal-install` depends on have the following (or similar) standard requirement:

> Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

Since we provide binary downloads of `cabal-install`, we should somehow comply with that. I think that the easiest way would be to add a "Legal" appendix to the manual listing the licenses of all of our dependencies. There should also be a way to generate such a page from a `.cabal` file in HTML or/and ReST format -- and since it's generally useful, this functionality should be part of `cabal-install`.