Convert certain HTML pastes to Markdown

[vgwidt]

This PR would allow certain HTML pastes to be converted to Markdown automatically in input forms where markdown is supported. This would be especially helpful for tables.

The first commit is a very simple, clumsy sample using html2md. When pasting, the handle_paste function gets the text/html value. It checks for a tag indicating it came from Excel, grabs the content from <table> to </table> and runs it through the html2md parse_html function to get the markdown equivalent.

I haven't looked too much into how robust html2md is for generating markdown (seems to cover original basics), but we might need to extend it or use html5ever to do it ourselves. This will take some looking into.

This requires rustflags=["--cfg=web_sys_unstable_apis"] due to the use of ClipboardEvent, but this results in dependencies being recompiled on each run. This would need to be fixed somehow before merging.

• Scanning the entire clipboard is not efficient, check the tags where the attributes should be to determine origin
• Fix data also being pasted as plain text (by means of oninput)
• Refactor code for use in all markdown editors
• Add other potential sources (i.e. LibreOffice)
• Make tables pretty with CSS
• Consider other types other than tables

[vgwidt]

It is worth mentioning that html is displayed (after sanitizing), so html tables do work without converting them to markdown.