GIF: support of animated GIF

[JeromeMartinez]

@jbkempf is trolling me about the lack of support of the most known video format, so quick implementation of its support.
Drawback: the file needs to be fully parsed for getting the count of frames / duration / frame rate. Files are usually not big but it is no more just the parsing of the header. Can be deactivated with --ParseSpeed=0 option.

Note that VLC support is poor, displaying just a still image (the first frame) and duration hardcoded at 10 seconds.

Rotating_earth_(large).gif

General
Complete name                            : Rotating_earth_(large).gif
Format                                   : GIF
Format/Info                              : Graphics Interchange Format
File size                                : 978 KiB
Duration                                 : 3 s 960 ms
Overall bit rate                         : 2 024 kb/s

Video
Format                                   : GIF
Format/Info                              : Graphics Interchange Format
Format profile                           : 89a
Duration                                 : 3 s 960 ms
Bit rate                                 : 1 937 kb/s
Width                                    : 400 pixels
Height                                   : 400 pixels
Display aspect ratio                     : 1.000
Frame rate                               : 11.111 FPS
Compression mode                         : Lossless
Bits/(Pixel*Frame)                       : 1.089
Stream size                              : 936 KiB (96%)

[jbkempf]

@jbkempf is trolling me about the lack of support of the most known video format, so quick implementation of its support.

I wouldn't dare! :)