Add command-line options

[PedroAS7]

We'll need command-line options to control how the program will run. We'll be using picocli (required).
The command-line options that will need to be handled:

• -i, --input-file: image to hide information in, or that has as a message hidden in it. Takes a string as input
• -o, --output-file: image with the message hidden in it (in case input file does not have it), or with the message removed (in case it does). Takes a string as input
• -m, --message-file: file containing the message to be hidden (in case input file does not have it), or where the message will be saved to (in case it does). Takes a string as input
• -b, --bits: number of bits to change in every pixel's colors. Takes an int $n, 1 \leq n \leq 8$ as input, where $n$ is a power of 2 (so, acceptable values are 1, 2, 4, 8)

[lentidas]

I think the input file, output file and message file are better expressed as picocli parameters (@CommandLine.Parameter() annotation) instead of options. The options are better suited for things like the number of bits or for activating encryption, eventually.