Documentation improvements and code cleanups for graph package

Expand the godoc documentation for the graph package.

Centralize DefaultTag in the graphs/tag package instead of defining it
twice.

Remove some unnecessary "config" structs that are only used to pass
a few parameters to a function.

Simplify the GetParentsSize function - there's no reason for it to take
an accumulator argument.

Unexport some functions that aren't needed outside the package.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>