[RFC] neovim startup time (~28% slower than vim)

I was curious to experiment with neovim's startup time. That feature was disabled and the PR adds it back in.

To test:

`nvim --startuptime /dev/stdout -c quit`
`vim --startuptime /dev/stdout -c quit`

My vim is stock 7.4 Huge version without GUI.

Considering how much code has been removed, I was surprised to see that it started slower. I tried re-compiling without the Debug flag and it produced similar results.

Perhaps it's too early to be thinking about optimizations, but I wanted to get a conversation started as well as getting this command-line option back in the source.