Support rasters which does not fit in memory

As I can see from source code rio-cogeo uses MemoryFile. Does it mean that this plugin is not suit for cases when source raster is not fit in memory?

[dionhaefner]

Yes, flake8 is very critical of lambdas (IMO to a fault, but who am I to judge). This should work though:

- dst_ctx = lambda: memfile.open(**meta)
+ dst_ctx = functools.partial(memfile.open, **meta)

If you’re not too paranoid about unclosed file descriptors in some edge cases I think it would also be perfectly fine to initialize the contexts first and enter them later:

if in_memory:
    file_ctx = MemoryFile()
    dst_ctx = memfile.open(**meta)
else:
    file_ctx = ...
    dst_ctx = ...

with file_ctx, dst_ctx as dst:
    ...

[drnextgis]

Good job @vincentsarago! Can we use contextlib2.ExitStack?