api: implement search

Reference: https://bugzilla.redhat.com/show_bug.cgi?id=1064118
Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
dnf-plugin: implement search

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
dnf-plugin: print description

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
api: find for fulltext in search

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>
dnf-plugin: search for fulltext

Signed-off-by: Igor Gnatenko <i.gnatenko.brain@gmail.com>