Make it possible to add fully custom columns to table definition

This adds a few different fields to the column SQL builder that were
once either calculated or hard-coded:
 * Import - so you can import the column from anywhere
 * Type - fully, rather than partially, modifiable to allow any naming
   convention
 * TypeFactory - fully, rather than partially defined

I also alphabetized some things because the OCD compels me.

generator/metadata/table_meta_data.go

@@ -2,9 +2,10 @@ package metadata
 
 // Table metadata struct
 type Table struct {
-	Name    string `sql:"primary_key"`
 	Comment string
 	Columns []Column
+	Imports []string
+	Name    string `sql:"primary_key"`
 }
 
 // MutableColumns returns list of mutable columns for table