Replace Tim Pope's vim-markdown with another one
Add "specify" to RSpec surround macros
We don't need tmux-navigator anymore

I switched from C-h|j|k|l bindings for tmux to prefix-h|j|k|l, meaning
these 2 don't clash anymore.
Remove submodules, Vundle will manage plugins
Replace command-t with :find again
Add syntax for .ejs templates
Bring back tpope/vim-markdown

plasticboy/vim-markdown was just too weird, and didn't function well,
and I realized I don't want markdown folding.
Remove seeing_is_believing plugin

I don't use it anymore, since it's for very rare use cases where I'm
writing an individual Ruby file, and I prefer puts in these cases.