ext.img_display: more efficient way to draw images

Instead of loading up w3mimgdisplay for each image and killing it
afterwards, we just open it once for the first image and keep it open.
It can receive any number of commands, so we can just keep writing them
into stdin.  Perhaps it even caches the images to save time, I didn't
test that yet.
core.fm: draw images later
Merge branch 'efficient_w3mimgpreview'
Changed email address in source code

Since lavabit.com ceased providing email services, I had to change my
address from hut lavabit com to hut lepus uberspace de.
config/rc.conf: make :find case-insensitive

The man page describes :find as case-insensitive.  It was also like this
before the implementation of the :scout command.  So let's change it
back until someone complains.
ext.direction: fix off-by-one error in select()
core.actions: better numerical argument to "mark_files"
core.actions: add numerical argument to toggle_visual_mode
ext.keybinding_parser: added key <S-DELETE>
ext.keybinding_parser: interpret keys like <383>

sometimes ranger's keybinding parser doesn't support whatever key you
want to use.  In that case, it would be useful to be able to specify
keys by their curses key code that can be taken from tools like
/doc/tools/print_keys.py.  <383> would then be interpreted as
<S-DELETE>, for example.
config/rc.conf: added :qa -> :quitall alias
Update vim_file_chooser.vim

Allow choosing multiple files.
Add ex command :RangerChooser.
Use the value of mapleader for the keybinding.
api.commands: improved tab-completion of :rename and others

The list of items that you tab through is now constrained to non-hidden
files and the order is the same as visible on the screen - starting from
the current file.