Postgres / SQL multistatement support

[bnkamalesh]

Is your feature request related to a problem? Please describe.
In case of Postgres migrations, for initial migrations where we setup schemas, it's common to use CREATE <table> IF NOT EXISTS. Though when creating user defined types, Postgres does not support this. The solution is to wrap type creations in try/catch and ignore duplicate exceptions, as shown below

DO $$ BEGIN
    CREATE TYPE my_custom_type AS ENUM(
        'Hello',
        'world'
    );
EXCEPTION
    WHEN duplicate_object THEN null;
END $$;

I saw there were issues filed for similar cases earlier as well.
ref:
#495 (review)
#1018

I understand the database specific implementations for parsing SQL and why golang-migrate resorted to a simple one based on ;

Describe the solution you'd like
Ability to specify a custom parser via config. e.g. the multistmt package can define an interface like, and accept any implementation.

type Parser interface {
 Parse(reader io.Reader, delimiter []byte, maxMigrationSize int, h Handler) error
}

Describe alternatives you've considered
If the migration returns an error with the specific error (by searching the error text), handle the migration explicitly from within the app, and then force a version upgrade. This seems cumbersome and prone to errors over time.