"list series" should allow filtering using a regular expression

The "list series" command should allow the return of a subset of series based on a regular expression similar to the way this is possible for select statements like "list series /stats\..*/i;".

Since issue #351 talks about introducing a system database I'm wondering if it would perhaps be possible to get rid of the special "list series" syntax altogether and instead take an approach similar to MySQL's information_schema and use something like "select name from system.db.testdb.timeseries where name =~ /stats\..*/".