Add support for trimmed GIFs, and other improvements

[dbburgess]

This includes a number of bigger changes, so if you want any of it broken off or merged in separately, just let me know and I can split this apart. I tried to avoid modifying the existing API so as not to break anything for anyone. Here is a summary of what changed:

• Add the ability to generate a GIF based off of a subset of the video (e.g., take from 0:30 to 0:45 and make a GIF of that).
• Add static convenience methods which allow for asynchronous generation.
• Modify the generator to use a more efficient method, decreasing generation time on average about ~40%.
• Update the documentation to make use of a standard header style.

Feedback and questions are, of course, welcome....Thanks for creating this library, it's saved me a lot of time!

[matthewpalmer]

This is awesome! 😍

I'll merge this in and do a cocoapods release for v1.2.

I'll also give you push access now :+1:

[dbburgess]

I'm glad you like it, thanks for the merge + release + access! :grinning: