Check pid/addr of target process was specified for subcommands

[sudo-suhas]

gops panics if anything other than help was invoked without specifying the pid/addr of the target:

λ gops stats
panic: runtime error: index out of range

goroutine 1 [running]:
main.main()
        E:/Installs/go-tools/src/github.com/google/gops/main.go:68 +0x48a

This adds a check and prints the usage if it was not specified.

λ go run main.go cmd.go stats
gops: pid/addr of target process not specified
gops is a tool to list and diagnose Go processes.

        gops <cmd> <pid|addr> ...
        gops <pid> # displays process info

...

All commands require the agent running on the Go process.
Symbol "*" indicates the process runs the agent.
exit status 1

[odeke-em]

Thank you @sudo-suhas for the fix. This looks like the result of a regression from this PR https://github.com/google/gops/pull/59/files#diff-7ddfb3e035b42cd70649cc33393fe32cL55
so basically reverting it.

@rakyll would have more context as to why we removed that check in that PR #59.

[rakyll]

Can you rebase this PR?

[sudo-suhas]

Done