Add base configuration

- Follow XDG standard: ~/.config and ~/.local folders
- Set environment variables and define aliases within ~/.config/shell
- Add installation script for easy setup
- Add README.md with info on the installation and general notes

.profile

@@ -0,0 +1,54 @@
+#!/bin/sh
+
+# Main setup file executed for all kinds of shells
+
+
+# Prevent loading ~/.profile twice in `bash`
+export PROFILE_LOADED=1
+
+
+# Basic utilities
+
+_command_exists() {
+    command -v "$1" 1>/dev/null 2>&1
+}
+
+_in_bash() {
+    [ -n "$BASH_VERSION" ]
+}
+
+_in_zsh() {
+    [ -n "$ZSH_VERSION" ]
+}
+
+_prepend_to_path () {
+    if [ -d "$1" ] ; then
+        case :$PATH: in
+            *:$1:*) ;;
+            *) PATH=$1:$PATH ;;
+        esac
+    fi
+}
+
+
+# Load configuration files common to all kinds of shells
+[ -f "$HOME/.config/shell/env" ] && . "$HOME/.config/shell/env"
+[ -f "$HOME/.config/shell/aliases" ] && . "$HOME/.config/shell/aliases"
+
+
+# Source ~/.profile_local, which holds machine-specific ENV variables
+[ -f "$HOME/.profile_local" ] && . "$HOME/.profile_local"
+
+
+# Load `bash`-specific configurations for non-login `bash` shells
+if [ -n "$BASH_VERSION" ] && [ -f "$HOME/.bashrc" ]; then
+    . "$HOME/.bashrc"
+fi
+
+
+# Put local executables on the `$PATH`
+_prepend_to_path "$HOME/.local/bin"
+
+
+# Ensure ~/.profile is loaded each time `bash` starts
+unset PROFILE_LOADED