Added dotfiles script

Necessary to make installing personal dotfiles a first class citizen
like many of the other install scripts. Having one's own dotfiles as
part of the machine setup helps ensure the shell is configured properly
especially when it comes to dealing with language specific dependencies
for Ruby, Rust, Node, etc. This reduces duplication within this project
-- and the macOS Configuration project -- from having to know all of
these details when settings can be defined within one's own dotfiles
once.

lib/options.sh

@@ -22,6 +22,8 @@ process_option() {
       bin/install_applications;;
     'x')
       bin/install_extensions;;
+    'df')
+      bin/install_dotfiles;;
     'np')
       bin/install_node_packages;;
     'rg')
@@ -41,6 +43,7 @@ process_option() {
       bin/install_app_store
       bin/install_applications
       bin/install_extensions
+      bin/install_dotfiles
       bin/install_node_packages
       bin/install_ruby_gems
       bin/install_rust_crates