Add -caption and -fontsize

[llimllib]

• Add -caption to set a single-line caption for the entire duration of the gif
• Add -fontsize, which sets the fontsize for the caption or the subtitles, whichever is used
• Document this feature and give an example
• Expand the complex_filters so they are a tiny bit more readable (still not very readable, try to come up with more better formatting)
• Log the bash version if we tell somebody they can't use this script

relates to #2