Add support for running a local debug server

[borgstrom]

Hi @ericdaugherty 👋

Thanks for this package! I added local debug server support for a project I've been working on and wanted to contribute it back.

I have a branch against the ask-cli project that adds first class support to use this via ask run, but in the mean time it is straight forward to issue your own access token and then start the server:

ask util generate-lwa-tokens --scopes alexa::ask:skills:debug
go run ./cmd/alexa-handler/ -debugServer -accessToken 'Atza|...'

Once running you can interact with your code via ask dialog.

Let me know if you're open to merging this and I can work on some tests and docs, and then we can submit a PR to ask-cli to get this hooked up.

[ericdaugherty]

I'd be happy to work with you to get this PR merged. It looks like it would be a helpful addition to the project.