Fully Genericize the Explain Command

[michaeltlombardi]

As with build, the explain command can be fully genericized for reuse without re-implementation in other projects such as PRM.

Acceptance Criteria

• Convert the var declaration in the explain command to a ExplainCommand struct
• Add the ExplainCommandI interface which must implement CreateCommand()
• Update CreateCommand(), preExecute(), validateArgCount(), flagCompletion(), and execute() to operate on the ExplainCommand struct, updating references from the variables to the struct's properties
• Remove the logic from CreateCommand which instantiates the docs file system; this should be passed when the struct is instantiated in main
• Remove any references to pct, genericizing where needed
• Ensure tests continue to pass