vifm: Add nsxiv as thumbnail previewer

Use <leader>t to start up nsxiv in thumbnail mode where it displays the
contents of the current directory.
You can do whatever you want with nsxiv, enlarge files with <Enter>,
flip them with | or - or delete them with D - the full bandwidth of
nsxiv is available.

If you mark pictures with m (or M) before exiting vifm will build a
custom filtered view containing only those files ready for further
filesystem operations with vifm.

Closes #11.

Also adds another layer of file previewer. If none of the other previewers
match (e.g. pdftotext for pdf, pandoc for docx, exa for directories, etc),
this viewer will be used and display the first couple hundred lines of a
file.

Essentially it means that unknown text files will be displayed using
this, but also binary files for which it displays the hexdump
automatically.

bootstrap/packages.tsv

@@ -56,6 +56,7 @@ dunst	Customizable and lightweight notification-daemon	R
 dust	A more intuitive version of du in rust	R	
 efm-langserver	General purpose Language Server	A	
 element-desktop	Glossy Matrix collaboration client — desktop version.	R	
+enca	Charset analyser and converter	R	
 entr	Run arbitrary commands when files change	R	
 evince	Document viewer (PDF, PostScript, XPS, djvu, dvi, tiff, cbr, cbz, cb7, cbt)	R	
 exa	ls replacement	R	

vifm/.config/vifm/scripts/vifm-default-viewer

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+if [ "$#" -ne 1 ]; then
+    echo "Usage: $0 filename"
+    exit 1
+fi
+
+# upper limit of lines to display for text files
+nlines=250
+# upper limit of bytes to display for binary files
+nbytes=2048
+# language of text files
+language=russian
+# output encoding for text files
+encoding=utf-8
+
+info=$(head -$nlines "$1" | file --mime -)
+charset=${info#*=}
+
+# shellcheck disable=2268
+if [ "x$charset" == "xbinary" ]; then
+    hexdump -e '"%08_ax: "' -e '8/1 "%02x " "  " 8/1 "%02x "' -e '" |" 16/1 "%_p"' -e '"\n"' -v -n $nbytes "$1"
+else
+    head -$nlines "$1" | enconv -g -L $language -x $encoding
+fi

vifm/.config/vifm/vifmrc

@@ -287,6 +287,11 @@ nnoremap <space>f :fzf<cr>
 nnoremap <space>c :fzfcd<cr>
 nnoremap <space>F :grep<space>
 
+" preview thumbnails of current folder
+" select thumbnails with m/M in nsxiv
+" to generate a filtered view on them
+nnoremap <space>t :!nsxiv -t %u -o %d<cr>
+
 " allows preview to work for normal view and single pane view
 noremap <silent> w : if &quickview && !layoutis('only')
                    \|     view
@@ -406,6 +411,8 @@ filextype *.html,*.htm
         \ chromium %f &,
         \ {Open with uzbl}
         \ google-chrome %f %i &,
+fileviewer *.html,*.htm
+        \ lynx -dump %c
 filetype *.html,*.htm links, lynx
 
 " Object
@@ -426,6 +433,8 @@ fileviewer *.gif
 
 " Images
 filextype *.bmp,*.jpg,*.jpeg,*.png,*.gif,*.xpm
+        \ {View in nsxiv}
+        \ nsxiv %f,
         \ {View in vimiv}
         \ vimiv %f,
         \ {View in sxiv}
@@ -549,6 +558,9 @@ fileviewer */
         \ exa --color always --tree -L2,
         \ tree -L 2,
 
+" use custom viewer script for rest
+fileviewer * vifm-default-viewer %c
+
 " Syntax highlighting in preview
 "
 " Explicitly set highlight type for some extensions