Add :Rschema command

Hey Tim,

I've implemented a :Rschema that when calling with a given table name
as an argument, Vim will jump to the table's position in the db/schema.rb file.
When no argument is given :Rschema will use the current file's model's table
name, and jump to it's position in db/schema.rb. Similar to how :R behaves when
called inside a model file.

Thanks,

Travis