Added support for tracking FK ID updates.

[barqshasbite]

In Django, you can update the raw ID of a ForeignKey relation
using the '*_id' field without assigning it a new model. This
needs to be logged by the audit framework. Currently it does
not detect these changes.

[codecov-io]

Codecov Report

Merging #202 into master will decrease coverage by 0.29%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master     #202     +/-   ##
=========================================
- Coverage   83.43%   83.13%   -0.3%     
=========================================
  Files          19       19             
  Lines         513      516      +3     
=========================================
+ Hits          428      429      +1     
- Misses         85       87      +2

Impacted Files	Coverage Δ
src/auditlog/diff.py	84.5% <50%> (-2.26%)	⬇️

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[aqeelat]

@hramezani do you think this and #280 are related?

[barqshasbite]

This change is no longer valid with the latest code base. A new pull request has been created to address the issue in latest master.