feat: add support for Mojave app notarization

[MarshallOfSound]

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• The changes are appropriately documented (if applicable).
• [] The changes have sufficient test coverage (if applicable).
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Summarize your changes:
Functionality comes from electron-notarize

We can't really add tests for this because it requires a legit apple ID and no CI system is updated to a high enough macOS / Xcode version to run tests 😆

// Example usage
package({
  osxSign: true,
  osxNotarize: {
    appleId: process.env.MY_APPLE_ID,
    appleIdPassword: process.env.MY_APPLE_ID_PASSWORD,
  }
})

[codecov]

Codecov Report

Merging #899 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #899   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines         641    671   +30     
=====================================
+ Hits          641    671   +30

Impacted Files	Coverage Δ
common.js	100% <100%> (ø)	⬆️
mac.js	100% <100%> (ø)	⬆️

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[malept]

Even if we can't test the actual use of electron-notarize, we should be able to test things like:

• it doesn't work when notarize is true but sign is not
• users can't pass nested fields like appBundleId and appPath to electron-notarize

[malept]

@MarshallOfSound this PR is dependent upon #900 due to the version of Node electron-notarize targets.

[malept]

no CI system is updated to a high enough macOS / Xcode version to run tests 😆

What are the minimum macOS/Xcode versions? Travis added Xcode 10.1 recently.

I made changes based off of my comments

[malept]

@MarshallOfSound I added tests (and moved some code around in the process). Could you take a look to see that the tests are the expected behavior?

[MarshallOfSound]

@malept Seems legit to me, thanks for picking that up 👍