Added script to categorize MDMA proportions and create stacked area plot 

I cleaned up the functions a bit more and broke up that massive function for better readability. I also rearranged the functions so that they have vertical hierarchy - the greatest abstraction is at the top while all dependencies for a function lie below. This is all thrown into the `scrape_data.R` file.

I also started the `clean_data.R` script to pre-process the data. We have a few variables to clean up due to leading and trailing white spaces and non-standard labeling. 

I have a working script to categorize MDMA proportions, but we definitely need to verify if this is correct or not. Feel free to test out `clean_data.R` -- it should end in stacked area plot in ggplot.