feat(readMetadata): add support for macOS target apps

[malept]

This should make it easier for installer modules for macOS targets to utilize this module.

[codecov]

Codecov Report

Merging #67 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #67   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          198       206    +8     
=========================================
+ Hits           198       206    +8     

Impacted Files	Coverage Δ
src/readmetadata.js	100.00% <100.00%> (ø)

---

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 b22887d...04cca6f. Read the comment docs.

[fcastilloec]

This is great, does this mean that electron-installer-dmg will be using this module? Or a new module is being developed for macOS based on -common?
Feel free to merge whenever you want

[malept]

This is great, does this mean that electron-installer-dmg will be using this module? Or a new module is being developed for macOS based on -common?

This was written for use in Electron Forge. electron-installer-dmg doesn't need to look inside the app metadata.