[restrict-plus-operands] rule doesn't support bigint

[novemberborn]

Repro

{
  "plugins": ["@typescript-eslint"],
  "rules": {
    "@typescript-eslint/restrict-plus-operands": "error"
  }
}

1n + 1n

Expected Result

No errors.

Actual Result

The operands are not numbers, but they're bigints which are still addable without type conversion.

Additional Info

Versions

package	version
@typescript-eslint/eslint-plugin	1.4.0
@typescript-eslint/parser	1.4.0
TypeScript	3.2.1
ESLint	X.Y.Z
node	X.Y.Z
npm	X.Y.Z

[webschik]

Hi!

I've created a PR with the fix #310.

[novemberborn]

This fixes 1n + 2n but not BigInt(1) + 2n or const foo = 1n; foo + 2n.

"bigint" should be mentioned in the error messages. (Yes I can PR that one. Not sure about the type checking for the other use cases.)