Added "emit_group_by_file" option

[hayama-junpei]

If "emit_group_by_file" is true, output the struct separately for each input query file, instead of being output collectively in one struct Queries.
The name of the struct is determined from the name of the input query file. For example, if the input file is authors.sql, the type Authors struct will be output.

Depending on the value of "emit_group_by_file", there are the following syntax differences when using it.

• "emit_group_by_file": false

  q := query.New(db)
  result, err := q.CreateAuthor(ctx, "Unknown")
  result, err = q.CreateBook(ctx, "my book title") 

• "emit_group_by_file": true

  authors := query.NewAuthors(db)
  result, err := authors.Create(ctx, "Unknown")
  books := query.NewBooks(db)
  result, err = books.Create(ctx, "my book title") 

When "emit_group_by_file" is true, the following two options are also affected.

• "emit_prepared_queries"

  • If true, outputs prepared statement support with a lower level syntax.

• "emit_interface":

  • if true, output a interface for each input query file, instead of being output a Querier interface.

[kyleconroy]

Thanks for putting together this PR, but I'm unlikely to merge this change. The reason is that it's possible to get a very similar output by using two separate packages. I know it's inconvenient, but I'd rather not add more options that complicate the code generation.