Clean up some syntax and add path checking to make DerpibooruDL a bit more robust as a CLI application

A few notable additions I've made:

- Put application code into main() so that importing the module outside __main__() doesn't execute it
- Leverage argparse's default options and switch arguments to save a few lines that were otherwise used toward testing for the existence of options
- Instantiate multiple variables in one line since they're all related to argparse
- Use the slightly more concise "if not apikey" syntax over "if apikey is None"
- Use path creation in case of the very likely possibility that the file destination doesn't exist