Add `limit` parameter

[jonysy]

Hi, the current limit when fetching species/plants is 20. It would be useful to have more control over this by having the limit parameter exposed as part of the API, which would be optional and would default to 20.

Example

https://trefle.io/api/v1/plants?token=<Token>&limit=6

[lambda2]

Hey @jonysy, unfortunately we're currently running Trefle in small servers, and can't allow too heavy responses as it will slow down everybody. So in order to ensure a quality service to all users, we prefer to limit to 20 results. For limiting results to less than 20 items, we could indeed add a limit parameter, I'll add that to the backlog !