Added to_pgxlsx and to_pgcsv support

[MedinsT]

No description provided.

[ebeigarts]

Would be really great if this could be used with any formatter - axlsx, xlsxtream, csv, etc.

module PostgresCopy
    def write_body(formatter, worksheet)
       ....
    end

    def to_csv
       ....
    end
end

and then we could use

class UserExport < Xport::Export
  include Xport::Axlsx
  include Xport::PostgresCopy # can be used for csv and excel

Also test would be very nice addition.