Added file install function.

Was originally removed in this commit (a895090ca9) and has been
restored in order to handle situations where we have only a single file
install.

For example, the MacOS Config project needs this function in order to
properly install the Pathogem (Vim) file.

lib/installers.sh

@@ -123,7 +123,6 @@ install_program() {
   local install_path=$(get_install_path "$program_name")
 
   if [[ ! -e "$install_path" ]]; then
-    printf "Installing: $install_path...\n"
     download_file "$url" "$program_name"
     mv "$MAC_OS_WORK_PATH/$program_name" "$install_path"
     chmod 755 "$install_path"
@@ -172,6 +171,23 @@ install_git_project() {
 }
 export -f install_git_project
 
+# Installs a single file.
+# Parameters: $1 (required) - URL, $2 (required) - Install path.
+install_file() {
+  local file_url="$1"
+  local file_name=$(get_file_name "$1")
+  local install_path="$2"
+
+  if [[ ! -e "$install_path" ]]; then
+    download_file "$file_url" "$file_name"
+    mkdir -p $(dirname "$install_path")
+    mv "$MAC_OS_WORK_PATH/$file_name" "$install_path"
+    printf "Installed: $file_name.\n"
+    verify_path "$install_path"
+  fi
+}
+export -f install_file
+
 # Downloads remote file to local disk.
 # Parameters: $1 (required) - URL, $2 (required) - File name, $3 (optional) - HTTP header.
 download_file() {