feat(cli): hc check and schema will print help msg when no subcommand

[j-lanson]

This MR resolves #105 and #106 issues pertaining to the replacement of unhelpful error msgs that Hipcheck emits when either check or schema subcommands are invoked without any arguments.

This is addressed simply by updating the type of the command field in the CheckArgs and SchemaArgs structs from an Option to just the underlying type. This conveys to clap that some subcommand must be parsed from the remaining arguments for things to move forward - if this does not happen, it will print the -h help msg.

The match statements associated with these command fields in main.rs are updated in accordance with this change.

[alilleybrinker]

LGTM!