Add HTTPS/HTTP2 server

[stramel]

Fixes #399

This will generate an HTTP2 server when using the --secure flag.

It will check the directory for a snowpack.cert and snowpack.key file and if not found will generate a local set to use.

Also, swapped the ETag over to use the weak option since we're using compression now. Missed in my previous PR #419

@FredKSchott I'm a bit unsure of how to get the files to output to the built files and wasn't sure where we wanted to look for/generate files. I was also not entirely sure the best way to resolve paths to files in Node. Those are the main things left to make this work.

A lot of this work is utilizing the work from servor.

NOTE: On Chrome, once I switched to the HTTP2 server, it seems like it was randomly not sending the if-none-match header. However, Firefox, continued to work fine.

[FredKSchott]

Looks great! I bet this has another noticeable perf impact for large apps

[stramel]

I modified the code slightly. Running --secure without key/cert,

1. Check the cwd for snowpack.crt and snowpack.key
2. If not found, generate key/cert in the node_modules/snowpack/assets folder as to not dirty the user's repo.
3. Check the node_modules/snowpack/assets for snowpack.crt and snowpack.key

I think this provides a bit more seamless solution. If we want to make the user more aware of the generated key/cert, I think we should like you said, require the user's consent before generating and generate into their cwd.

Personally, I prefer the more seamless approach but either is fine with me.

[stramel]

One concern that I have using the certify.sh file is that it is only tested on Linux and Mac OSX. I was able to run it on my Windows machine when utilizing the Linux sub-system, although, I'm unsure if it would work for users that are running standard cmd prompt.

Also, I didn't add the section of changing the process id for installing the rootCA automatically in the keychain. This felt a bit awkward so instead, I left the snowpackCA.crt in the assets dir for manual installation if a user wishes to do so.

For better cross-platform usage, we may want to look into using devcert instead.

[FredKSchott]

@stramel If we're worried about the stability of this (especially the fact that it can't run on Windows) lets pull that logic out for now and just give explicit commands that the user can run if they need to generate. IE:

$ snowpack dev --secure
! No HTTPS credentials found! Missing Files: snowpack.crt, snowpack.key 

You can automatically generate credentials for your project via either:
  - devcert (no install needed): npx devcert-cli generate localhost
    https://github.com/davewasmer/devcert-cli (no install required)
  - mkcert: mkcert -install && mkcert localhost
    https://github.com/FiloSottile/mkcert (install required)

# exit

I think this may even be the best long-term solution as well. I mentioned elsewhere, but based on what I've seen I don't think we have the resources to maintain a custom integration that handles automatic generation & connecting that to your internal key store all perfectly within Snowpack.

[stramel]

@FredKSchott I did a bit of formatting on the error message. Feel free to clean it up however you see fit.

Also, I opened an issue on devcert-cli to be able to specify the output file names. davewasmer/devcert-cli#5 This way we can just have a single command and be up and running for snowpack.

[FredKSchott]

LGTM!

[FredKSchott]

Thanks for tackling, this is great!

Assuming you've been able to test this running locally, feel free to merge when ready