feat(cli): feally featured RevealJS template

[jeertmans]

This adds an option for every possible RevealJS option. Error messages are also improved.

RevealJS version is bumped to latest (4.4.0).

[jeertmans]

This PR fixes most of the issues mentioned @lucmos in #65, namely:

• Framerate, Smooth Transition, and Keybindings: those can be achieved using a custom template, see manim-slides convert --help;
• First animation is skipped: this was fixed by muting the audio, since most browsers prevent playing audio without the user first interacting, read this SO thread for more details;
• Page Numbers: you can now configure them using config options.

For the Flashing background, I think this is a problem related to internet connection being slow or not, but this might need more investigation.

Therefore, I am closing #65 with this PR.