ragtag: Enable ragtag for markdown, disable <C-_>

ragtag: Enable ragtag for markdown, disable <C-_>

Ragtag initializes only for certain file types so disable lazy load of
ragtag by vim-plug. Since markdown is not in this list explicitly call
RagtagInit().

Also disable the old <C-_> shortcut and use ragtag's <C-X>/ instead.