CUID port for go doc added

[akshayvadher]

Since the readme didn't have a Go port, I only discovered two ports had already been created for Go when I came to PR. My bad.

Anyway, I created one at https://github.com/akshayvadher/cuid2

I have added all the histograms, char frequencies, and collision tests.

I made the syntax and structure as similar as possible so that reviewers could review it easily and make future changes easier.
Instead of removing the first char from the hash, I returned it as it was; the collision and histogram tests were failing (to test out that IDs are correctly generated and tests are correctly testing).

The entropy generator uses global variables (ENV variable keys).

Changes:

• Instead of ^[0-9a-z]+$, the test CUID function checks for ^[a-z][0-9a-z]+$ for more accurate result
• For histogram test, instead of dropping the first two chars (.slice(2)), I am dropping only the first for more accurate testing