Add instant markdown plugin

https://github.com/suan/vim-instant-markdown

[sudo] gem install redcarpet pygments.rb
[sudo] npm -g install instant-markdown-d

.gitmodules

@@ -34,3 +34,6 @@
 [submodule "vim/bundle/syntastic"]
 	path = vim/bundle/syntastic
 	url = https://github.com/scrooloose/syntastic.git
+[submodule "vim/bundle/instant-markdown"]
+	path = vim/bundle/instant-markdown
+	url = https://github.com/suan/vim-instant-markdown.git

vim/bundle/instant-markdown

@@ -0,0 +1 @@
+Subproject commit 54421032cb7b21faf8be8e139f8d83c7fee16080