Subscribers endpoints

[Mantas97]

Issue: https://github.com/mailerlite/mailerlite-nodejs/issues/2

Description:

• Reusing making request functions from MS SDK. These are used on MS 1-2 years so why reinvent a wheel?
• Tried isomorphic-unfetch package at first, but moved to axios. I think axios has nicer error handling with .catch(() => {})

Testing:

npm install tsc cp .env.example .env & add you API key Use script from examples directory like this one

[LanaAb]

@JuliusB can you review this when you have a minute