Fix vim addon local vimrc for opening directories

Hi Marc, I came across another one of your useful plugins, thanks.  

Here's a small patch:  My local vimrc was not getting loaded if I opened a directory with vim.  I'm now triggering the local vimrc lookup to occur after VimEnter, rather than at script load time, and that seems to fix it.

This commit calls SourceLocalVimrcOnce from the VimEnter
event, enabling localvimrc's to be loaded even if vim is
just opening a directory.