Feature - Geo types in model generation for PostGIS

[fabianhjr]

No description provided.

[lexmag]

I think these types don't really belong to the Phoenix and should be declared in model manually.

[chrismccord]

Thanks for the PR. As @lexmag said, I think this is outside of the concern of phoenix and it requires a dependency on the Geo project, so it's best to handle these fields outside of the generators.