Add filter for layer

[c-w]

This lets us for example exclude all features that are at a too high level (e.g. country or continent) by explicitly specifying all the layers that we care about and only getting results from those layers.

Try it live:

• Include all "Boston" matches
• Only include "Boston" at county level
• Only include "Boston" at county and locality level

The list of all layer is as follows:

features=# select count(*), layer from features group by layer order by count desc;
 count  |     layer
--------+---------------
 212672 | locality
 118725 | localadmin
  90320 | neighbourhood
  47008 | county
   4996 | region
   4423 | campus
   1666 | microhood
    375 | macrocounty
    351 | macrohood
    228 | country
    138 | borough
    109 | macroregion
     41 | disputed
     39 | dependency
      8 | continent

Knowledge of this list is useful if we only want to exclude particular layers since the features database doesn't have a concept of layer hierarchy at this point. A description of the layers can be found in the Who's on First documentation.

[erikschlegel]

LGTM