Proposal: errors option

**Purpose:** There are recoverable errors in syntax error, e.g. duplicate names, invalid escapes in RegExp, and etc. ESLint stops all linting if it encountered those recoverable errors. However, ESLint should be able to work.

**Proposal:** Adds `errors` option to espree.

```js
const ast = espree.parse(code, { errors: true })
console.log(ast.errors) //→ Array of recoverable errors.
```

`acorn` reports errors by `this.raiseRecoverable()` method if the error is recoverable. If the `errors:true` option is given, espree collects recoverable errors into `ast.errors` property instead of throwing exception.

Then ESLint can report the `ast.errors` to users.