Add support for pmtiles output format (#55)

* add pmtiles.hpp from github.com/protomaps/PMTiles [#10]

* tippecanoe main writes pmtiles output. [#10]

* detect output format using suffix
* after mbtiles is done writing, replace with pmtiles based on map/image tables.
* add method to write_json for writing json sub-object.

* tippecanoe-decode reads pmtiles input. [#10]

* tile-join reads and writes pmtiles. [#10]

* pmtiles test suite for decode and tile-join [#10]

* add base GitHub CI action for compiling and test suite.

* update pmtiles.hpp with z>15 fix

* Fix some ordering problems with pmtiles decode

* Pmtiles should also pass the raw tiles tests

* Eradicate spaces from tileset metadata JSON fields

* Eradicate spaces from more test fixtures

* Update more tests

* Pmtiles tests pass now too

* Remove unnecessary sort (and make indent)

* Update changelog

* The allow-existing test for pmtiles needs -o, not -e

* Declare --allow-existing to be unsupported for pmtiles.

It was always a bad idea even for mbtiles.

Co-authored-by: Brandon Liu <bdon@bdon.org>

tests/minzoom/out/-z6.json

@@ -4,7 +4,7 @@
 "description": "tests/minzoom/out/-z6.json.check.mbtiles",
 "format": "pbf",
 "generator_options": "./tippecanoe -q -a@ -f -o tests/minzoom/out/-z6.json.check.mbtiles -z6 tests/minzoom/in.json",
-"json": "{\"vector_layers\": [ { \"id\": \"in\", \"description\": \"\", \"minzoom\": 0, \"maxzoom\": 6, \"fields\": {} } ],\"tilestats\": {\"layerCount\": 1,\"layers\": [{\"layer\": \"in\",\"count\": 7,\"geometry\": \"Point\",\"attributeCount\": 0,\"attributes\": []}]}}",
+"json": "{\"vector_layers\":[{\"id\":\"in\",\"description\":\"\",\"minzoom\":0,\"maxzoom\":6,\"fields\":{}}],\"tilestats\":{\"layerCount\":1,\"layers\":[{\"layer\":\"in\",\"count\":7,\"geometry\":\"Point\",\"attributeCount\":0,\"attributes\":[]}]}}",
 "maxzoom": "6",
 "minzoom": "0",
 "name": "tests/minzoom/out/-z6.json.check.mbtiles",