Support COVERALLS_PARALLEL environment variable

[vtjnash]

No description provided.

[fingolfin]

Looks OK to me so far. I assume this will be squashed once completed? It seems there are still some issues to be ironed out, based on the updates this PR already received ... :-). I'll be happy to review this again once it has stabilized.

[fingolfin]

Not a reason to hold up this PR, just wondering, but: is there any reason to do it like this, and now how the README suggests to do i? I.e.,

env:
  global:
    - COVERALLS_PARALLEL=true

[vtjnash]

Just that it confuses testing to set it globally. We similarly don't use the suggested script either.

[codecov-io]

Codecov Report

Merging #250 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   90.22%   90.31%   +0.08%     
==========================================
  Files           6        6              
  Lines         348      351       +3     
==========================================
+ Hits          314      317       +3     
  Misses         34       34

Impacted Files	Coverage Δ
src/coveralls.jl	81.08% <100%> (+0.79%)	⬆️

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[vtjnash]

I'll be happy to review this again once it has stabilized.

Yeah, was just making all of the edits in the GitHub editor, so I could only do one file at a time. If it wasn't clear, it should be done now. I planned to squash-and-merge.

[fingolfin]

Still looks good (assuming it'll be squashed, as you said)