Allow to use the Installer programatically

[unsegnor]

Allow using Installer programatically regardless of the NODE_ENV variable value.

I was looking for a way to install chromedriver as a javascript function.
I did this in the beginning of the file:

let previousNodeEnv = process.env.NODE_ENV
process.env.NODE_ENV = 'test' //this is needed to get the ChromedriverInstaller from chromedriver/install
process.env.DETECT_CHROMEDRIVER_VERSION = true
const ChromedriverInstaller = require('chromedriver/install')
process.env.NODE_ENV = previousNodeEnv

It would be nice if I could avoid those env variables and call the method with parameters.
Or at least avoid having to set the NODE_ENV, which is the goal of this PR :)

[giggio]

The problem is that the installer module is not part of the exposed interface, so this is why it is hidden behind a flag.
If I was to expose it then I would not be able to change this file freely, anymore. For example, I would not be able to rename install to something else.
I'd prefer to reflect on what would be exposed, and if it is worth the effort.