Consolidate all Context code into Command

[meatballhat]

(wild generalizations warning) The Context struct exists for reasons mostly related to the use of stdlib flag and has a concept of "lineage" that forms a tree with other Context nodes. I would like to see this lineage relationship move to Command given the existence of .Commands for traversing outward, and for Command to be the source of truth for values that are fetched via the type-specific flag accessor functions like .String(string) string, .Int(string) int, .Bool(string) bool.