dired-preview

This is a simple package to automatically preview in a side window the
file at point in Dired buffers.  Preview windows are closed when they
are no longer relevant, while preview buffers are killed if they have
not been used for other purposes beside previewing.

Enable the `dired-preview-mode' in the current Dired buffer or
`dired-preview-global-mode' for all Dired buffers and then perform
the regular up/down motions.  Those will trigger the preview.

The previewed file is displayed in a side window if its size is below
the number specified in the user option `dired-preview-max-size'.
Previews are shown subject to a small delay, per ther user option
`dired-preview-delay'.

Files matching the `dired-preview-ignored-extensions-regexp' are
not previewed, though a preview window is still displayed if the
user option `dired-preview-ignored-show-ignored-placeholders' is
non-nil.  This is to avoid windows jumping in and out of focus as
the user moves between files.

[ In the future, we may find ways to quickly preview any file type
  without affecting the performance of Emacs.]

I took inspiration for `dired-preview' from the now unmaintained
`peep-dired' package by Adam Sokolnicki: <https://github.com/asok/peep-dired>.
My original plan was to volunteer to maintain `peep-dired` but I
decided to write it my own way: it was easier for me, plus I can
implement only what I consider necessary without upsetting existing
users.