Add codecov limit on coverage reduction.

[cjcscott]

This adds an additional codecov target which should fail if a PR reduces test coverage by more than 0.5%. This is arbitrarily configurable, but this seemed like a level of reduction which should allow large PRs with partial test coverage while prompting consideration if tests are totally absent.

This follows on from #123 , which fixed an issue with coverage calculation introduced in #97 that would've been caught by a check like this.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (bf7cbc7) 72.52% compared to head (ddbc189) 72.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #124 +/- ## ======================================= Coverage 72.52% 72.52% ======================================= Files 142 142 Lines 19088 19088 Branches 3056 3056 ======================================= Hits 13844 13844 Misses 4479 4479 Partials 765 765 ```

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