Add ability to set pixel format for the following functions/methods and classes:
 - ``write_videofile()`` from ``VideoClip`` class.
 - ``FFMPEG_VideoReader`` class.
 - ``ffmpeg_read_image`` function from 'ffmpeg_reader.py'
 - ``FFMPEG_VideoWriter`` class from 'ffmpeg_writer.py'
 - ``ffmpeg_write_video`` function from 'ffmpeg_writer.py'
 - ``ffmpeg_writer_image`` function from 'ffmpeg_writer.py'
 - ``VideoFileClip`` class.

Add the functionality to change the pixel format when creating a gif.
But this new feature is only available when ffmpeg is used.

This new feature change ``write_gif`` from 'VideoClip.py'. As well
``write_gif_with_tempfiles`` and ``write_gif`` from 'gif_writers.py'

Add the ability to set the pixel format when create a gif using
tempfiles.