Improvements for getApi()

[Collin-St]

Solution for #252

Changes

index.js - getApi()

• Check if ngrokClient is defined, return if so. Else, Call getProcessUrl to get the process URL for port 4040, and create a new instance of NgrokClient. Fallback to null.

src/process.js

• Include getProcessUrl which spawns a child process and makes the platform appropriate query for port 4040's process URL.

ngrok.d.ts

• Update getApi() type definition to return a nullable promise.

test/ngrok.guest.spec.js

• Update "getting internal api wrapper" test case to call getApi() with async/await.
• Include additional assertion "should be an instance of NgrokClient" .

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Tests run successfully on Mac, Windows 10, and Linux. Open to any improvements!