Multi-schema support and diffing two databases

Summary

• Diff any schemas, not just public
• Diff two databases instead of a database and DDL
• New interface for generating plans (Generate). GeneratePlan is now deprecated.

What's Changed

• Switch interfaces to accept conn pools by @bplunkett-stripe in #90
• Concurrent schema fetching by @bplunkett-stripe in #91
• Fix potential deadlock bug by @bplunkett-stripe in #92
• Set max connections for plan cmd by @bplunkett-stripe in #93
• Use connection pool when validating migration plan by @bplunkett-stripe in #95
• Add support for fetching non-public schemas. by @bplunkett-stripe in #96
• New GeneratePlan interface by @bplunkett-stripe in #98
• Wire schemas through SQL generation by @bplunkett-stripe in #99
• Add exclude schemas by @bplunkett-stripe in #100
• Multi-schema support by @bplunkett-stripe in #104
• Database schema source (allow comparing two databases) by @bplunkett-stripe in #107
• Update CLI with multi-schema support and database schema source by @bplunkett-stripe in #108

Full Changelog: v0.5.0...v0.6.0