Allow Avram::Credentials to build unix sockets

[jwoertink]

If you need to build a unix socket, but want to use the Avram::Credentials object, it's a bit messy. I think we can make this easier to work with.

Here's an example of how to connect with a unix socket

# this ENV must be set otherwise it defaults to the current user's name on the system
# export PGDATABASE=my_db_development
Avram::Credentials.parse("postgresql:///my_db_development?host=/var/lib/postgresql/.s.PGSQL.5432&dbname=my_db_development")

I think you can also set a username and password using

Avram::Credentials.parse("postgresql://user:pass@/my_db_development?host=/var/lib/postgresql/.s.PGSQL.5432&dbname=my_db_development")

We would just need to make sure these methods account for the change

avram/src/avram/migrator/runner.cr

Lines 49 to 59 in f5b000c

	def self.drop_db(quiet? : Bool = false)
	DB.connect("#{credentials.connection_string}/#{db_user}") do |db|
	db.exec "DROP DATABASE IF EXISTS #{db_name}"
	end
	unless quiet?
	puts "Done dropping #{Avram::Migrator::Runner.db_name.colorize(:green)}"
	end
	end
	def self.create_db(quiet? : Bool = false)
	DB.connect("#{credentials.connection_string}/#{db_user}") do |db|

Related: #776