#18: Adding Support for --config/-c Flag

[lyu4321]

This PR closes #18.

This PR adds a --config/-c flag option that accepts a JSON config file as an argument. The contents of the file replace command line arguments. For example, using a file named config.json that looks like

{
  "input": "./site",
  "output": "./build"
}

The command

java -jar out\artifacts\ssgApplication_jar\ssgApplication.jar --config config.json

Would replace

java -jar out\artifacts\ssgApplication_jar\ssgApplication.jar --input ./site --output ./build