Improve startup time by organizing into ftplugin and autoload folders

[fent]

Hi, I was profiling my vim startup time and noticed that vim-markdown-preview is taking unusually long compared to other plugins

146.843  019.587  019.587: sourcing /Users/fent/.vim/plugged/vim-markdown-preview/plugin/vim-markdown-preview.vim

This is from using the --startuptime flag. I then realized I hadn't opened a markdown file, and so the plugin shouldn't be sourced. That's the first part of this PR.

The 2nd part is organizing the main functionality into the autoload folder. When we open a markdown file, we don't wanna fully load the plugin until we use it.

After that optimization, the startup time for the script looks like

133.973  000.129  000.129: sourcing /Users/fent/code/vim/vim-markdown-preview/ftplugin/markdown.vim