Use vim's autoload feature

Use vim's autoload feature to reduce the startup time.

Without this, command-t takes 66ms of vim startup time (no doubt due to the ruby interpreter) which is annoying if vim is loaded for something that's not going to use command-t (e.g. git commit). After autoload, it takes something likes 0.15ms of startup time (measured with vim --startuptime).

While it seems to work fine, this was the first time I've looked at vimscript so you may want to redo this one...