Add ``--search`` option

The option does substring search on name, attribute name, and meta-description for all the words specified on command-line. So for example ``nix-env -qa --search python amqp`` gives you expected result.

I believe it should have short alias, but I'm not sure which one. Both ``-S`` and ``-s`` are used. May be ``-F`` (i.e. find or filter)