Variable length support

I'm implementing a protocol based on EDIFACT. In the specification of the protocol it is specified on an per element basis if this an fixed or variable length field.

I would be happy to implement this feature and create a pull request, but I'm not sure where this property would go in the current structure where type and length specification are provided by the PICTURE class. The Picture class is provided to the methods in the EdiTextWriter class that add the padding to the written values so adding this property to the Picture class would enable me to switch in this writer's methods to enable or disable padding at that point. As far as I see there is no notion of fixed vs variable length variables in the PICTURE clause notation, therefore this issue might also be related to #23).

Start by reading the Picture class and the EdiTextWriter methods that add padding, then review the EDIFACT fixed- versus variable-length requirement and related issue #23. Done means the design supports specifying length behavior per element and writing variable-length values without inappropriate padding.

Written by the indexing model from the issue text.