use stringi for aggregation and feature merging

Should be ready to go, though there are some issues worth discussing before merging into master, namely:

- should there be a master script that just calls the component scripts in order
- where in the pipleline to insert the trump script
- possibility of converting these scripts to take STDIN arguments using R on command line, instead of forcing users to run in interactive mode (this would help solve the issue of where to insert the trump script; users could run it at any point, either before or after merging in features, demographics, etc).
- next steps:
    - writing the `merge_demographics` and `merge_genealogy` scripts
    - writing the feature reduction / optimization script