Release 5.0 tracker

[mfdebian]

This issue will be used as a tracker for the 5.0 release (as a replacement for #203)

Key changes looking forward a 5.0 version:

• Update async strategy to use Promises (feat(lib): updates code to use es6 and promise based async  #231)
• Drop support for Node < 6 (feat(lib): updates code to use es6 and promise based async  #231)
• Use standard style throughout the library (feat(lib): updates code to use es6 and promise based async  #231)
• Add newer versions of Node to the test matrix (chore(ci/cd): add newer node versions to test matrix #243)
• Add tests and ensure compatibility for Express 5.x
• Drop support for Express 2.x
• Remove layout support
• Address current open issues/PRs
• Rename master as main?

There were already PRs that address things from the previous tracker such as 5ee89a1 and 361bdd7, we'll need to review them and check how to rebase/add them to our current 5.0 branch. I'll be doing that on the following weeks.

I'd also like to rename master to main, let me know what you think!

[bjohansebas]

Drop support for Express 2.x

express 2.x o express 3.x?

[bjohansebas]

I also recommend that we drop support up to version 18!