* new function extract_from_esummary() for extracting like-named elements
from a list esummary records (e.g. get all "Title" fields from a list of
PubMed esummaries)
* Support for `cmd` option in entrez_link (breaks backward compatibility)
* Allows discovery of external links from and use of web_history
* New helper function linkout_urls to get URLs form external links
* Support for 'by_id' mode for entrez_link. Pass a vector of IDs to
entrez_link, (optionally) get a list on elink objects back (one per ID)
* New web_history object makes using NCBI Web History features easier
* All of these changes documented in new vignette
* Han Guangchun added as contributor for his pull requests
* New tests, minor bug fixes and extended documentation
