add vim mail syntax highlighting (#137)

vim/ftplugin/himalaya-msg-read.vim

@@ -1,9 +1,11 @@
 setlocal bufhidden=wipe
 setlocal buftype=nofile
 setlocal cursorline
+setlocal filetype=mail
 setlocal foldexpr=himalaya#shared#thread#fold(v:lnum)
 setlocal foldmethod=expr
 setlocal nomodifiable
+syntax on
 
 call himalaya#shared#bindings#define([
   \["n", "gw", "msg#write"      ],

vim/ftplugin/himalaya-msg-write.vim

@@ -1,7 +1,9 @@
 setlocal cursorline
+setlocal filetype=mail
 setlocal foldexpr=himalaya#shared#thread#fold(v:lnum)
 setlocal foldmethod=expr
 setlocal startofline
+syntax on
 
 augroup himalaya_write
   autocmd! * <buffer>