Implement the missing command option

[rhettc]

Added support to supply SQL when invoking this command.

Also includes the prompt feature mentioned in #71. I figured that as long as this command is stepping on the original implementation, it might as well have better feature parity.

[msakrejda]

Thanks for the pull request, but I think continuing separate implemenations of pg:psql across the toolbelt and this plugin seems like a really bad idea. I think we need to resolve that issue before adding any more functionality here, probably by adding --set hooks to the toolbelt that this plugin can shim into, and leaving the main pg:psql command in the toolbelt.

[rhettc]

Yeah - I think that's a good plan. However, currently, users that are installing this plugin are losing functionality because even thought the documentation was copied from the original, it doesn't implement the features advertised. It's a frustrating user experience, and after the second time I found myself scratching my head in a matter of weeks,I decided to offer this fix. Respectfully, I don't see this as adding a new feature, just making the add-in comply with the requirements.

[will]

The pg-extras pqsl no longer shadows the toolbelt's.