bugs: allow searching by maintainer, inc. project

- bugs: improve best match finder - bugs: add ``--find-by-maintainer`` option - bugs: add support for filtering targets by StableRequest - bugs: auto extend maintainers with projects So a new super invocation now is: ``` pkgdev bugs --find-by-maintainer arthurzam@gentoo.org,-ppc@gentoo.org,-java@gentoo.org --projects --filter-stablereqs ``` Resolves: https://github.com/pkgcore/pkgdev/issues/157