Bug 15630 - Make Edifact module pluggable

 Bug 15630 - Make Edifact module pluggable

Koha's EDIFACT module works great for many European vendors,
but does not work will for US vendors, which have a much different
interpretation of 'standard'. In fact, each vendor may require
different arrangements of values in EDIFACT messages. It would be
impossible to encompass all these requirements within Koha's EDIFACT
module itself. Instead, we should allow the module to be pluggable, so
versions of the module can be developed for vendors that require EDIFACT
messages that don't conform to the standard set by Koha's EDIFACT
module.

Test Plan:
1) Apply this patch
2) Run updatedatabase
3) Enable Koha plugins
4) Install the Edifact stub plugin available at
   https://github.com/bywatersolutions/koha-plugin-edifact-stub
5) Edit the EDI Vendor account, assign the plugin to a Vendor EDI account
6) Test EDI functionality ( ORDER, INVOICE ), there should be no errors
   or changes to the EDIFACT message input or output

Signed-off-by: Jason DeShaw <JDeShaw@cityoffargo.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>