Allow updating the value of an enum column

[direction]

Something along the lines of

class Table
  ... 
  def change(old_value, new_value)
    @connection.execute "UPDATE #{@connection.quote_table_name @name}
    SET value = #{@connection.quote old_value} WHERE value = #{@connection.quote new_value}"
  end
end

in https://github.com/howaboutwe/enum_table/blob/master/lib/enum_table/schema_statements.rb#L66

[oggy]

@direction

I dig it - except I'd probably call it rename to feel a little more analogous with {add,remove,rename}_column.

Feel like sending a pull request w/ test? :)