Add a wrapper around the iframe, add height input, fix grammar

[kankaristo]

Sorry to make so many changes in one PR, but I got a bit carried away.

This PR:

• adds a wrapper element around the iframe, mostly to allow automatic height calculation (defaults to 16:9 aspect ratio)
• removes the use of CSS classes
  • pro: the iframes continue to work even if you remove the plugin
  • con: more verbose code in the Markdown document
• adds a text input for setting the iframe's height (defaults to automatic 16:9 aspect ratio)
• fixes https://github.com/FHachez/obsidian-convert-url-to-iframe/issues/1 (was broken for me too)
• improved grammar in README.md and in user-facing messages (use of a/an, proper capitalization)
• minor code cleanup (only for parts that were changed anyway)

[FHachez]

Hey, I'm sorry, I wasn't very active recently. I'll have a look tonight.