add plugin storage table to schema; add ability to clear plugin data

schema/versions/pgsql/101.sql

@@ -0,0 +1,11 @@
+begin;
+
+create table ttrss_plugin_storage (
+	id serial not null primary key,
+	name varchar(100) not null,
+	owner_uid integer not null references ttrss_users(id) ON DELETE CASCADE,
+	content text not null);
+
+update ttrss_version set schema_version = 101;
+
+commit;