Add Unique Constraint with tests

[marcotas]

For column schema:

table.string('email').unique();
// can provide a name for the constraint
table.string('email').unique('some_name');

For the table schema:

table.unique(['name']);
// or
table.unique(['name', 'email']);

• For create operation:

create table "test" ("email" varchar(255) unique)

• For update operation:

alter table "test"
alter column "email" type varchar(255)
alter column "email" add constraint "test_email_unique" unique ("email");

• Drop operation

table.dropUnique('name_unique');

Generates:

alter table "test" drop constraint "name_unique"