Apply 'desmos-graph' css class to graph element

[Nigecat]

This allows for easier custom styling of the render output using Obsidian's CSS snippets.

For instance, if the user wished to center all graphs in the page content, they could add the following to a snippet:

/* Horizontally center the graph in the page content */
.desmos-graph {
    display: block;
    margin-left: auto;
    margin-right: auto;
}

• [x] Implemented
• [x] Changes are backwards compatible
• [x] Docs have been updated (including an example of usage)
• [x] Latest master commit has been merged and rebuilt with changes

This is a re-merge of #55 after #56 since the commit message was incorrect and misleading.