geojson: add support for "external" json encoders/decoders

[paulmach]

From the included readme changes

If GeoJSON encoding/decoding performance is critical consider using a
third party replacement of "encoding/json" like github.com/json-iterator/go

This can be enabled with something like this:

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

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

CustomJSONMarshaler = c
CustomJSONUnmarshaler = c

The above change can have dramatic performance implications, see the benchmarks below
on a 100k feature collection file:

benchmark                             old ns/op     new ns/op     delta
BenchmarkFeatureMarshalJSON-12        2694543       733480        -72.78%
BenchmarkFeatureUnmarshalJSON-12      5383825       2738183       -49.14%
BenchmarkGeometryMarshalJSON-12       210107        62789         -70.12%
BenchmarkGeometryUnmarshalJSON-12     691472        144689        -79.08%

benchmark                             old allocs     new allocs     delta
BenchmarkFeatureMarshalJSON-12        7818           2316           -70.38%
BenchmarkFeatureUnmarshalJSON-12      23047          31946          +38.61%
BenchmarkGeometryMarshalJSON-12       2              3              +50.00%
BenchmarkGeometryUnmarshalJSON-12     2042           18             -99.12%

benchmark                             old bytes     new bytes     delta
BenchmarkFeatureMarshalJSON-12        794088        490251        -38.26%
BenchmarkFeatureUnmarshalJSON-12      766354        1068497       +39.43%
BenchmarkGeometryMarshalJSON-12       24787         18650         -24.76%
BenchmarkGeometryUnmarshalJSON-12     79784         51374         -35.61%