Implement EXCLUDE constraint for PostgreSQL

[KES777]

This pull request will allow to generate next statement

ALTER TABLE subnets ADD CONSTRAINT subnets_subnet_udx  EXCLUDE USING gist ( subnet inet_ops with && ) WHERE state = 0;

with next syntax:

$sqlt_table->add_constraint(
    name       =>  'subnets_subnet_udx',
    type       =>  'exclude',
    fields     =>  'subnet',
    expression =>  'subnet inet_ops with &&',
    options    =>  { using => 'gist', where => 'state = 0' },
);

[KES777]

Q: What is wrong with perl-5.8 here? A: Travis-CI team answer:

The command that's failing is "perl Makefile.PL && make test" and it seems to be failing to move beyond some questions posed during the "make". What does it do? Please review the process here, since the issue appears to be coming from your code. You may also try reproducing the problem with our Docker images. https://docs.travis-ci.com/user/common-build-problems/#Troubleshooting-Locally-in-a-Docker-Image

[rabbiveesh]

Hey, this is great, but it's incomplete + i can't push to your branch. I'm gonna close this and start a new PR on top of your commit, and i'll credit you in the changelog.

Thanks so much for contributing, even if there was a 6 year gap til it got picked up!

[KES777]

@rabbiveesh Thanks for this.