Add Support for ColumnAttribute in Dapper.Contrib

[soundaranbu]

Hi, first of all, thanks for the great library!
I've a scenario where the table column names are in snake_case.
I know Dapper.Contrib will work when we name the C# class properties in the same way (snake_case). But that's against the .NET naming conventions and doesn't look good. In order to keep the table columns in snake_case and the C# property name in PascalCase we need an attribute ColumnAttribute similar to TableAttribute which will map the C# class properties to the table column names

or if there's switch that will convert PascalCase to snake_case on query generation is also fine.

[soundaranbu]

Just now realized there was a plan already #722 . Any updates on this? @NickCraver