Feature Request: Update the centering when opening a new `:Startify`

[ndavd]

Would it be possible to implement a way to re-render the header when opening Startify in a new window? Maybe a function startify#update() who knows.... I'm just looking for a way to recenter it.

For example, open nvim in fullscreen, then do a vsplit.. This is the result:

[ndavd]

As it turns out it isn't necessary, if someone wants to update startify one only has to redefine the header/footer again and call startify#insane_in_the_membrane(0) to reload it. However, one issue that arises is that someone might want to reload the center of startify in a vsplit, but the startify#center() function is currently getting the width by &columns. I propose that we change that to winwidth(0) instead. The behavior will be the same but when on a split it will take into account the window's width instead of the screen's width. I'll make a PR