Enhancement of ncbi_search

I added a couple of arguments to `ncbi search` and reorganized the code to avoid redundancy. The main changes are the ability to add custom Entrez search terms and enhanced handleing of the `getrelated` option.   It looks quite a bit differnt, but most of the core functionality is the same code. I made a Rmarkdown file that explains the changes I made and compares the output of the old and new functions. Check out https://github.com/zachary-foster/ncbi_search_update_test