Add structured option to pelias geocoder

[knaaptime]

the pelias geocoder has a new(ish) structured endpoint that's currently in beta, but works quite nicely. The modifications to an existing pelias call are pretty small (hit the search/structured endpoint instead of /search, and urlencode a handful of specific parameters concatenated together instead of one big query string) so I'd be more than happy to take a first pass at a PR if it's of interest. In the contributing guide I notice there's some discussion about how to test against more specific params of a specific geocoder and whether those params should be hardcoded, so maybe thats an area i could use some pointers :)

[knaaptime]

also, thanks for all your work in this project!!