Add ninja modes to hoon.vim

We added ninja modes to hoon.vim.  They are turned off by default; to enable, add "let g:hoon_ninja = 1" to your .vimrc.  Then, in insert mode, typing "//" will put you in ninja mode, where if you type, e.g., "bartis" (or "brts"), it will convert it to "|=".  Typing "??" puts you in ajnin mode, where typing "|=" yields "bartis", and typing "/?" puts you in jnn mode, where typing "|=" yields "brts".