feat: `Obsidian paste` can handle html to markdown conversion

[neo451]

There's two mean jobs of obsidian app's paste, one is attachment pasting (#77), the other is to turn html into valid markdown.

we can generalize the concept into just paste, 1) check the clipboard 2) determine type 3) handle attachment or html

• first ship one version that just use pandoc if available (?)
• handle all major clipboard tools
• require"obsidian".paste
• give the paste a default mapping <leader>p or gp or <C-S-v>?
• deprecate Obsidan paste_img