feat(typescript): export `ApplicationFunctionOptions` type

addresses https://github.com/probot/probot/pull/1405\#issuecomment-731488088

fix: remove deprecation messages about no longer used environment var…

…iables "IGNORED_ACCOUNTS" and "DISABLE_STATS"

test: adapt for latest supertest

fix: load app function only once when using createNodeMiddleware (pro…

…bot#1432)

fix: load app function only once when using createNodeMiddleware (pro…

…bot#1432)

fix: `createProbot() without options

feat: `createProbot()`, `createNodeMiddleware(app, { probot })`. Depr…

…ecates `getOptions`, `createNodeMiddleware(app, { Probot })` (probot#1431)

fix: `app.route()` with `(app) => {}` app function (probot#1430)

fix: `app.route()` with `(app) => {}` app function (probot#1430)

test: remove `test/fixtures/test-private-key.pem` from gitignore