WIP: Add -G to main_group

[sgillies]

This option puts the file_in_arg handler into legacy filename mode: /vsi*/ paths will work and so will database connection strings that would not otherwise pass validation.

This resolves #871 and may possibly make obsolete #1079.

To be merged after #1346.

Example:

$ rio -G info /vsizip/tests/data/white-gemini-iv.zip/white-gemini-iv.vrt
/Users/sean/code/rasterio/rasterio/__init__.py:226: NotGeoreferencedWarning: Dataset has no geotransform set. Default transform will be applied (Affine.identity())
  s = DatasetReader(fp, driver=driver, **kwargs)
{"blockxsize": 128, "blockysize": 128, "bounds": [0.0, 768.0, 1024.0, 0.0], "colorinterp": ["grey", "grey", "grey"], "count": 3, "crs": null, "descriptions": [null, null, null], "driver": "VRT", "dtype": "uint8", "height": 768, "indexes": [1, 2, 3], "mask_flags": [["all_valid"], ["all_valid"], ["all_valid"]], "nodata": null, "res": [1.0, -1.0], "shape": [768, 1024], "tiled": true, "transform": [1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0], "units": [null, null, null], "width": 1024}

[sgillies]

#1351 makes this one unnecessary. Closing.