Fix prototype pollution

[evangelion1204]

Fixed prototype pollution by ignoring __proto__ in sections and keys.

Steps to reproduce
payload.ini

[__proto__]
polluted = "polluted"

poc.js:

var ini = require('multi-ini')

var parsed = ini.read('./payload.toml', { encoding: 'utf8' })
console.log(parsed)
console.log(parsed.__proto__)
console.log({}.__proto__)
console.log(polluted)

> node poc.js
{}
{ polluted: 'polluted' }
{ polluted: 'polluted' }
polluted

[coveralls]

Coverage increased (+0.1%) to 95.111% when pulling 4617804 on fix-prototype-pollution into f494134 on master.