More browser support & port forwarding

[mrfrase3]

Description

Hey, I'm the maintainer of duino.app, an open-source browser-based Arduino programmer. I was looking at having to code browser-side uploading manually until I spotted that you had since added browser support!

This PR addresses some things to make the browser implementation a little more feature complete so I could use it in my application.

There are three main things done in this PR:

1. Allow passing/forwarding through an existing node-serial/web-serial port, in case the calling application has already gotten the user to select the serial port and/or is using it for other things like serial monitoring
2. Fill out some of the missing node-serial features
3. Close the connection more gracefully

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

The port forwarding is a pretty advanced use case, it's nice that it's there, but I'm uncertain if it should be "supported" by documenting it.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Changes have been manually tested (please provide information on test platform using the fields below)

Test / Development Platform Information

• Operating system and version
  Manjaro Linux
• Avrgirl Arduino version
  latest
• NodeJS version
  12.x
• Arduino Board being used
  Uno, Mega

[coveralls]

Coverage remained the same at 46.689% when pulling 18c6b1b on duinoapp:browser-port-forwarding into 4c88107 on noopkat:master.