Support a minimum query length

[orangejulius]

Very short autocomplete queries (one or two characters) are often not particularly helpful. Many autocomplete interfaces support a minimum input length, below which a query won't be sent to the underlying search backend, and instead a message (something like "enter 1 more character to search...") will be displayed.

We should support something similar.

[missinglink]

This is implemented in the code but not configurable: https://github.com/pelias/leaflet-plugin/blob/master/src/core.js#L23