Add gifs to the README to illustrate functionality

[ericphanson]

I used ProgressMeter as a proving ground for my new package Asciicast.jl which intends to make it easy and maintainable to have animated examples in github READMEs. (The package is not registered, but will be in a couple days).

This PR turns some of the README examples into Asciicast blocks, by adding the syntax {cast = "true"} to some of the julia code blocks. Then Asciicast.cast_readme(ProgressMeter) executes them (using the same sandboxes as Documenter) and saves the results as gifs (using agg) to the assets directory, then uses pandoc to insert the image links back into the README.

This has the benefit of being easily maintainable, but the drawback that it uses pandoc's opinionated github-flavored markdown formatting, which causes a somewhat large diff here.

I am not sure if this is desired here, but since I was trying out Asciicast with ProgressMeter anyway, I figured I'd make a PR.

[timholy]

This is pretty cool! I think it's a clear win in terms of usability for newbies. I guess one question is whether this will increase the package size: do the assets get downloaded when you install the package?

[giordano]

do the assets get downloaded when you install the package?

Yes, a snapshot of the entire repository is installed, but all the images in this PR are less than 100 kB each (Edit: apart from one, which is 225 kB 😅).