Re-organize the shells' initialization logic

- unify ~/.config/shell/aliases.sh and ~/.config/shell/utils.sh
  into a common ~/.config/shell/init.sh
- split the contents of ~/.config/shell/aliases.sh
  into the folder ~/.config/shell/aliases.d
- split the contents of ~/.config/shell/utils.sh
  into the folder ~/.config/shell/utils.d

.config/shell/utils.d/files.sh

@@ -0,0 +1,66 @@
+# The utilities defined here make working with files and folders easier
+
+
+# List the $PATH variable, one element per line
+# (if an argument is passed, grep for it)
+path() {
+    if [ -n "$1" ]; then
+        echo $PATH | perl -p -e 's/:/\n/g;' | grep -i "$1"
+    else
+        echo $PATH | perl -p -e 's/:/\n/g;'
+    fi
+}
+
+
+# Show folders by size
+disk-usage() {
+    if [ -n "$1" ]; then
+        _dest="$1"
+    else
+        _dest=.
+    fi
+    \du --human-readable --max-depth=1 $_dest 2>/dev/null | sort --human-numeric-sort --reverse
+}
+
+
+# Search all files in a directory and its children
+lsgrep() {
+    ls --almost-all --directory . ./**/* | uniq | grep --color=auto -i "$*"
+}
+
+# Make a directory and cd there
+mcd() {
+    test -n "$1" || return
+    mkdir -p "$1" && cd "$1" || return
+}
+
+
+# Extract any compressed archive or file
+extract() {
+    if [ -f "$1" ] ; then
+        case "$1" in
+            *.tar.bz2) tar xjvf "$1" ;;
+            *.tar.gz)  tar xzvf "$1" ;;
+            *.tar.xz)  tar xvf "$1" ;;
+            *.bz2)     bzip2 -d "$1" ;;
+            *.gz)      gunzip "$1" ;;
+            *.tar)     tar xf "$1" ;;
+            *.tbz2)    tar xjf "$1" ;;
+            *.tgz)     tar xzf "$1" ;;
+            *.zip)     unzip "$1" ;;
+            *.Z)       uncompress "$1" ;;
+            *.7z)      7z x "$1" ;;
+            *)         echo "'$1' cannot be extracted automatically" ;;
+        esac
+    else
+        echo "'$1' is not a file"
+    fi
+}
+
+mktar() {  # out of a directory
+    tar cvzf "${1%%/}.tar.gz" "${1%%/}/"
+}
+
+mkzip() {  # out of a file or directory
+    zip -r "${1%%/}.zip" "$1"
+}