Doc changes

All cosmetic changes to make Walker look more awesome.
- Moved documentation links near to top of README
- Added console screenshot
- Added/updated to godoc-style package comments to our subpackages
- Some wording changes in the console (also for the sake of the screenshot)
- Uncommented keys `walker.yaml` -- I liked them commented because it made it super clear that you don't need a walker.yaml at all to run walker, but it also made it hard to read the file. Decided just to uncomment the keys so it looks nice and clear
Looks similar to what they did here: https://gowalker.org/github.com/shenwei356/util/bytesize