update usage handler and handle no args

[pallavJha]

The PR
- updates the usage handler
- adds example input for -n and -y flags

$ ./sandy 
Usage: ./sandy [OPTIONS] command
  -h	Print Usage.
  -n value
    	A glob pattern for automatically blocking file reads.
    	For example, "/etc/password.txt" or "*.txt".
  -y value
    	A glob pattern for automatically allowing file reads.
    	Expected format is same as -n.

$ ./sandy -h
Usage: ./sandy [OPTIONS] command
  -h	Print Usage.
  -n value
    	A glob pattern for automatically blocking file reads.
    	For example, "/etc/password.txt" or "*.txt".
  -y value
    	A glob pattern for automatically allowing file reads.
    	Expected format is same as -n.

Fixes #1

[craigmulligan]

Cheers!