Adds a new [.mbtiles](https://github.com/mapbox/mbtiles-spec/blob/master/1.3/spec.md)
backend, without the grid support. Uses extensive tile content
detection, i.e. if the content is gzipped, png, jpeg, gif, webp.
From CLI, can be as easy as adding a path to a directory that contains a
.mbtiles file (works just like pmtiles support)
```bash
# All *.mbtiles files in this dir will be published.
# The filename will be used as the source ID
martin ./tests/fixtures
```
From configuration file, the path can be specified in a number of ways
(same as pmtiles)
```yaml
mbtiles:
paths:
# scan this whole dir, matching all *.mbtiles files
- /dir-path
# specific mbtiles file will be published as mbtiles2 source
- /path/to/mbtiles2.mbtiles
sources:
# named source matching source name to a single file
pm-src1: /tmp/mbtiles.mbtiles
# named source, where the filename is explicitly set. This way we will be able to add more options later
pm-src2:
path: /tmp/mbtiles.mbtiles
```
Fixes#494
