Create interface for command line flags

[artenator]

The emulator currently supports logging the output of each NES instruction to stdout, and also to a file. But, the only way to currently do this is to manually alter the code.

There are two functions in main.go

The RunInstruction function takes an instruction and a boolean as arguments, where the boolean defines whether or not to log to stdout.

The initLogOutput() function pipes stdout to a log file.

We should create command line options like --log to pipe instruction output to stdout, and --logfile asdf.log for piping the logs to a file.

Golang provides a simple command line argument parsing package flag.
https://gobyexample.com/command-line-flags