extract add parameter for exact matches

[ehwenk]

Add option to extract_ functions to only allow exact matches - versus the default which uses partial string matches.

There are instances where partial matches return too much data:

For instance

searching on trait = "leaf_area" returns data for "leaf_area" and "leaf_area_ratio"
searching for taxonomic_status = "accepted" also returns data for "genus accepted" and therefore more taxon names than are sought.

The default is still for partial matches, but there is now a parameter "partial_matches_allowed" which can be set to FALSE.

This branch also includes a different bug fix for extract_data

[dfalster]

Nice work! Just checking you want partial_matches_allowed=TRUE as default?