Publish two versions of windows binaries

[springmeyer]

We currently only support vs2015 per https://github.com/mapnik/node-mapnik#windows-specific. A common convention with node-pre-gyp modules is to allow users to pass --toolset=v140 to opt-into binaries built with vs2015. Because node-mapnik only supports vs2015 we provide binaries for both npm install (no custom flag) and npm install --toolset=v140. This keeps back compatibility with node-mapnik versions pre v3.6.0.

Background at #756.

[codecov-io]

Codecov Report

Merging #757 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #757   +/-   ##
=======================================
  Coverage   96.23%   96.23%           
=======================================
  Files          42       42           
  Lines        8810     8810           
=======================================
  Hits         8478     8478           
  Misses        332      332

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4fa5ab6...1f19181. Read the comment docs.

[springmeyer]

Merging this now. Will not republish binaries since @flippmoke already fixed this problem manually for v3.6.0. Merging this will protect us from the problem next release.