correctly unmarshal elements with a type tag

[paulmach]

as described in #51 the "quick hack" to find the type does not work if the the element has "tags":{"type":"route"} like relations have.

This PR finds the tag using an unmarshal, it does impact performance.

benchmark                            old ns/op     new ns/op     delta
BenchmarkChange_UnmarshalJSON-10     1099707       1331954       +21.12%

benchmark                            old allocs     new allocs     delta
BenchmarkChange_UnmarshalJSON-10     4880           5327           +9.16%

benchmark                            old bytes     new bytes     delta
BenchmarkChange_UnmarshalJSON-10     273049        252964        -7.36%

I was able to get it down to +7% speed but much more memory using json.Decoder.Token. I opted for the simpler approach as it allowed the plugging in of "customer unmarshallers".

If performance is a concern consider using jsoniter or similar like

import (
  jsoniter "github.com/json-iterator/go"
  "github.com/paulmach/osm"
)

var c = jsoniter.Config{
  EscapeHTML:              true,
  SortMapKeys:             false,
  ValidateJsonRawMessage:  false,
  MarshalFloatWith6Digits: true,
}.Froze()

osm.CustomJSONMarshaler = c
osm.CustomJSONUnmarshaler = c