add 1-dim probability map shader to chunkflow neuroglancer command

[eschombu]

This adds a user-controllable colored shader to 1-dimensional probability map neuroglancer layers. The color can be specified in the chunkflow command parameters with the following syntax:

chunkflow \
  ...
  neuroglancer -i img,seg,glia[color=red],mito

Above, the glia variable contains the glia probability prediction at each voxel in the chunk. When this layer is added via the _append_probability_map_layer, a new color parameter takes its value from the parameters specified inside the [...] brackets. Extracting this parameter value is implemented in the neuroglancer operator's __call__ method by checking for comma-separated key=value pairs within such square brackets. This mechanism can be reused for other layer-type-specific keyword parameters in the future.

[xiuliren]

this is a very useful feature to have!