Add Depth First Search in Go

[rzuckerm]

To request a new code snippet, please fill out the following:

Project name: Depth First Search
Project link: https://sampleprograms.io/projects/depth-first-search
Language: Go

The project link must be a link to a project on the Sample Programs Website project list.
If you would like to add a new project to the Sample Programs Website,
please first make a pull request to the Sample Programs Website.