Add brief description in README.
Allow query and id_list to be vectors in arxiv_search/count
test-clean: as.numeric -> as.vector

 - for stripping attributes from arxiv_search results
Add omit_attr function; use in test-clean
Handle empty results

  - arxiv_search() was throwing an error
README: Add note that you don't need an API key
Fix bug in clean_links for case with no links in input
_Now_ that bug in clean_links is fixed.

 - Also add weird test for case when id_list values are syntactically
   correct but not found
Add to ToDo list: weird id_list values
arXiv identifiers come back as URLs; turn them into plain IDs
Add beginnings of a vignette