Add object-sync example

[GretaCB]

Per https://github.com/mapbox/node-cpp-skel/pull/44, continuing to expand examples. Super rough right now.

• This object-sync example builds off the sync standalone example, but implements it as part of a Class
• Not sure the hello framing is very intuitive, so pondering better object use-cases/metaphors
• Gradually removing src/hello_world.cpp, and replacing parts of it with this example, and pondering what pieces are best to break apart into other examples

cc @mapbox/core-tech

[codecov-io]

Codecov Report

Merging #48 into master will decrease coverage by 4.76%. The diff coverage is 90%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   97.61%   92.85%   -4.77%     
==========================================
  Files           5        5              
  Lines         126       70      -56     
==========================================
- Hits          123       65      -58     
- Misses          3        5       +2

Impacted Files	Coverage Δ
src/object_sync/hello.hpp	0% <0%> (ø)
src/module.cpp	100% <100%> (ø)	:arrow_up:
src/standalone/hello.cpp	100% <100%> (ø)	:arrow_up:
src/object_sync/hello.cpp	92.59% <92.59%> (ø)
src/hello_world.hpp

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[GretaCB]

Update

Per https://github.com/mapbox/node-cpp-skel/pull/48/files/a55288299aa6393e99257c35556aa38c8ccfdd4d#r123389311, I've started to rethink the design of this object_sync example. I'm running into some issues related to how to access member vars of an instance from within a function, how that plays into the Init function and generally what is happening with the constructor calls.

I've added a few more comments per chat with @flippmoke , but I'm still a little wobbly with the concepts (ex: Singleton). Would love to run the new design by you, @springmeyer , if you have some time on Monday.