Add .schema command

[tzzed]

Proposal

This is a proposal of .schema command to show the CREATE statements matching pattern.
The command shows all tables as default or shows the table name option.

Expected:

genji> .schema  [table_name]
CREATE TABLE foo (
    bar text NOT NULL,
   baz text NOT NULL
);

CREATE INDEX idx_foo_bar ON foo (bar);

CREATE TABLE bar (
    baz text NOT NULL,
);

Motivation

Like the .schema command of sqlite this is a good command to see the schema of all tables.