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
In a better world we'd make this a string: so user could write "20MB" and get the right number of bytes. Maybe in the future ...