add db schema for sql schema format

[kortirso]

Hi, I used SolidErrors for one of my projects with ruby schema format and everything was fine

but later I tried to integrate it to another project with sql schema format, so such structure.sql schema is required for gem's work, maybe this will be helpful for somebody else who use sql format

[fractaledmind]

I get the idea, and I like it, but this sql schema is PG specific, and the gem is db agnostic. We would need to find a way to make sql schema creation dynamic and db agnostic