feat(diff): compare config in both ways

[ta2edchimp]

Implements the two way diffing on

eslint-diff-rules config1 config2

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This PR still includes its predecessor's commit — so I guess it (again) should be rebased after #58 got merged (why does this not get resolved automatically??) ...

[codecov-io]

Current coverage is 100.00%

Merging #60 into master will not affect coverage as of 291b23a

@@            master     #60   diff @@
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  Files            7       7       
  Stmts          137     154    +17
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit            137     154    +17
  Partial          0       0       
  Missed           0       0       

Review entire Coverage Diff as of 291b23a

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[sarbbottam]

LGTM, I will leave it to you to merge the PR, I guess this one is built on top of the other.

[ta2edchimp]

Yep. I'll merge #58 tomorrow, rebase and merge this one afterwards and build upon those with the original PR #50.

Now off to 😴 soon

[ta2edchimp]

This PR still includes its predecessor's commit — so I guess it (again) should be rebased after #58 got merged (why does this not get resolved automatically??) ...

I guess I have to explicitly cherry-pick the commits into branches for future PRs of this kind.

[ta2edchimp]

For the record, I tweaked bin/diff's tests a bit to reflect the two way diffing.