Propose option to search common names

[mjwestgate]

One limitation of galah is the need to supply taxonomic identifiers as scientific (Latin) names. It would be nice if users were able to supply common names instead. According to the 'bulk species lookup' API documentation (on this page):

"To allow the lookup to consider common names, include a "vernacular":true value in the JSON map: json {"names":["Grevillea"],"vernacular":true}"

...in which case this could be a simple change to search_taxa. Alternatively, if the API settings only return common names, it might be better behaviour to create a whole new function for this purpose (e.g. search_names)