Make it easier to use Snowplow Micro in development

Kind of a follow-up to this discussion in product-intelligence#266.

Problem

The current workflow to make use of Snowplow Micro in development is fairly long (10 steps), a bit complicated, and requires you to alter several lines of code in the application codebase.

Solution

We can make it a bit simpler by including those altered lines of code directly in the codebase and introducing some sort of flag (an ENV variable, maybe?) which will make use of those lines of code only if we’re also in the development environment.