Add write_area_to_binary and get_pixel

[akash-akya]

Added new generic function write_area_to_binary which extracts a region to raw binary in a single NIF call as an optimization.

write_to_binary, write_to_tensor, to_list and others now use this function so there might be slight performance gain.

get_pixel internally just calls write_area_to_binary with width & height 1 and returns a 1D list of band values.

# the value will be based on the image band_format and colorspace
{:ok, [r, g, b]} = Image.get_pixel(image, 10, 10)

[akash-akya]

@kipcole9 can you take look? I'll add docs, tests and refactor a bit later.

[akash-akya]

Addresses #153

[kipcole9]

Awesome work as always Akash, I'll give this a spin over the next couple of days. Thanks very much.