Adding captioning

This should be up to date with the master branch, and added the `-t` flag to put captions on the bottom of GIFs. This requires the `--enable-libass` with they install `ffmpeg`. **[Documentation](https://trac.ffmpeg.org/wiki/HowToBurnSubtitlesIntoVideo)**.

The download URL wasn't working with the `curl` script, so I updated it to reflect what worked for me. Could have just been on my end, though, so I can remove that change if you'd like.

**Major changes**
Lines `24-31` are a function to produce a properly formatted `.srt` file for captioning.
Lines `69-79` bring my branch up to the latest version of `master`.
Lines `81-91` add the caption if the `-t` flag is called. The caption needs to be in quotes in order to work properly. Text will wrap automatically for long strings on the completed GIF. 

The options list as well as other arguments have all been updated as well.