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]

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

@@           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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