Rio-mbtiles

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- Consolidate --append/--overwrite into one option and make appending the
default.

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- Add a --covers option, taking a quadkey, which limits the output to tiles
that cover the quadkey's tile (66).

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- Add --oo (open) options and --wo (warp) options like those of gdalwarp. These
allow control over, for example, overview level of TMS datasets and the
number of threads used internally by GDAL's warper.
- Add a --cutline option that takes an optional path to a GeoJSON
FeatureCollection (62). No pixels outside the cutline shape(s) will be
exported and no tiles outside the cutline will be generated.

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- Support appending to or updating existing mbtiles files (59).
- Add an optional progress bar based on tqdm.
- Add concurrent.futures and multiprocessing implementations of process_tiles()
(54).

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- Missing support for RGBA input and output (PNG only) has been added (26).
Using the options --format PNG --rgba with rio-mbtiles will create RGBA PNG
tiles from the first 4 bands of the input dataset.
- Output tile size has been made an command option. The default remains
256 x 256 (44).

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- Write out empty tiles at the lowest zoom levels when the reverse transform
fails during our attempt to skip empty tiles entirely (41).
- Avoid modifying rasterio windows in place as they have been made immutable
in new versions of Rasterio (43).