This commit adds a `NERDTreeCreatePrefix` setting that can be used to
prefix the `:edit` command that is used to create the NERDTree tree
window. Defaults to "silent", meaning that out of the box the window
will be created with "silent edit".
Users may wish to configure this to produce other effects. For example,
`NERDTreeCreatePrefix` can be set to "silent keepalt keepjumps" in order
to make NERDTree create its window with "silent keepalt keepjumps edit".
This can be used to create an effect analogous to the `g:netrw_altfile`
setting in netrw. An example of why you might want to do this is
described here:
https://github.com/tpope/vim-vinegar/issues/25
I'm not using vim-vinegar myself, but I am using something like it here:
https://github.com/wincent/wincent/blob/3efaa8fa50895/roles/dotfiles/files/.vim/plugin/mappings.vim#L60
And having `NERDTreeCreatePrefix` enables me to map "-" to show the
current file in context, and `^-6` to jump back to it.
Add 2 new classes and move code into them from autoload:
* NERDTree. Each nerdtree buffer now has a NERDTree object that holds
the root node and will old other util functions
* UI. Each NERDTree object holds a UI object which is responsible for
rendering, getting the current node, etc
Still a fair few methods to sort through in autoload (many of which will
end up in the above classes) - need sleep though.
On some new versions of vim these files were being loaded before the
main NERD_tree.vim which was causing errors as dependencies werent
loaded in time. Move the classes into lib - so vim wont try to load them
until we tell it
