feat: support options.externalKey

[vicanso]

I want to get the external key by another way, such as http header, query string etc.

[coveralls]

Coverage remained the same at 100.0% when pulling 0c2c594 on vicanso:master into fd8acf2 on koajs:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0c2c594 on vicanso:master into fd8acf2 on koajs:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 16c1217 on vicanso:master into fd8acf2 on koajs:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 16c1217 on vicanso:master into fd8acf2 on koajs:master.

[kherock]

Is there any reason this feature hasn't landed? It's the main thing preventing me from switching from koa-generic-session. I need a bit more control over exactly what info is stored in the session cookie.

[dead-horse]

sorry for the delay, I'd like to have the signature of opt.externalKey like:

opts.externalKey = {
  get(ctx) { return externalKey; },
  set(ctx, value) { },
}

@vicanso do you have time to modify this PR?

[vicanso]

@dead-horse I have deleted the repo, and pulled a new request #149 . Please review it, thanks.