Triggering lvimrc load on BufEnter

Currently, the localvimrc loads the lvimrc files on `VimEnter`, `BufNewFile`
and `BufRead`. That should be enough for loading and switching files inside the
same window, but when working across windows this won't work for loading a
buffer which is already displayed on another window.

This commit switches the behavior to load on `BufEnter`, which should be less
efficient but work across windows.