Handle custom predicates for GBIF searches

[hannahlowens]

Suggested during review by @damianooldoni.

"This is the classic dilemma while programming high level functions: the balance between being user friendly and flexibility.

Personally I would allow users to make their own query by means of ellipsis .... In this way your function is powerful but at the same time user friendly: GBIF/rgbif experts can specify their own predicates while other users just benefit of the basic functionality and so everybody is happy."