Introduce token cache

[snobu]

We need a token cache that:

• Saves the access token (preferably encrypted) to local storage (~/.destreamer/token.cache or something like that to be consistent with every other project out there that needs to save state).
• Decodes the token on every run, looks at the expiration date and decides if we still need to pop up a browser window to acquire a new token or we can proceed without user interaction.

This will dramatically improve the user experience since within the span of one hour (Azure AD issues tokens with 1-hour lifetime) users can download videos without having to log back in.

Decode sample:

import * as jwt_decode from 'jwt-decode';

const token = 'eyJ0eXAi...';
const decoded = jwt_decode(token);

let now = Math.floor(Date.now() / 1000);
let exp = decoded['exp'];
let timeLeft = exp - now; // seconds

Stretch goal:

• Can we get an access token for https://*.microsoftstream.com just by doing an Open ID connect / OAuth 2.0 dance using the default system browser (drop puppeteer altogether).

Implementation in progress. See tokencache branch for status.

[lukaarma]

It's working great, the only issue we may have is if the token expires mid-download but for that there is already #43

[snobu]

Closing since token cache has been implemented. Tracking /refreshtoken implementation via https://github.com/snobu/destreamer/issues/43