Add a IPython magic command for searching tables within a specific DB

[takuti]

Since this command fetches all databases and tables, it can be slow even if a database name is given.

[takuti]

Correction - the given argument is NOT a database name, and it's a regexp for table name:

%td_tables [table_pattern]

The code searches all table names that match to the pattern across all DBs.

If we need to improve its efficiency by focusing on a specific DB name, it should be a new function as:

%td_tables [db_name] [pattern]