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/aliases.d/python.sh

@@ -0,0 +1,25 @@
+# Make working with Python more convenient
+
+
+_command_exists() {
+    command -v "$1" 1>/dev/null 2>&1
+}
+
+
+# Interactive shells
+alias py='python'
+alias bpy='bpython'
+alias ipy='ipython'
+
+
+if _command_exists poetry; then
+    alias pr='poetry run'
+fi
+
+
+if _command_exists pyenv; then
+    alias pyvenvs='pyenv virtualenvs --bare --skip-aliases'
+    alias pyver='pyenv version'
+    alias pyvers='pyenv versions --skip-aliases'
+    alias pywhich='pyenv which'
+fi