Providing and checking typescript types

Features

• providing type definitions

Tooling

• Type checks using tsc as part of pretest script (incl. some minor code changes to make tsc run successful)
• enable code coverage check on CI (using tap)
  • code coverage is submitted to https://codecov.io/
• npm run release (using np)
  • .npmrc remove git-tag-version=false, add sign-git-tag=true

Docs

• README.md: badges for npm, dependencies, codecov
• doc comments for ExitCode

Other Stuff

• Update devDependency @types/node to v13.1.1
• test.command.js: assertStdout less branches (to test) when checking stdErr

v0.2.0...v0.3.0