add possibility to set additional data parameters in resource

[PetrDlouhy]

This PR adds possibility to specify additional export parameters otherwise specified by custom admin export form.

I am planning to add some more docs and tests.

[PetrDlouhy]

@timthelion What do you think about this modification? Are you OK with the usage of JSONField? Other fields doesn't use it even if they contain JSON data, but I expect that originates in time when JSONField wasn't part of Django.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 56.61%. Comparing base (7a9ec54) to head (8bcc25f).

Files with missing lines	Patch %	Lines
import_export_celery/models/exportjob.py	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
- Coverage   56.76%   56.61%   -0.16%     
==========================================
  Files          10       10              
  Lines         377      378       +1     
  Branches       57       57              
==========================================
  Hits          214      214              
- Misses        160      161       +1     
  Partials        3        3              

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