Optional dependency of typescript.

[allex]

make the typescript not a strong required dependence. allow customize typescript implements like:

// rollup.config.js
// ...
plugins: [
    ts2({
      typescript: require('@allex/typescript'),
      tsconfig: resolve('tsconfig.json'),
      check: false
    })
    // ...
]

[ezolenko]

Looks good, could you fix the codacy errors? I'll merge it then.