Add example to download a file

Author: milancurcic

example/download_file.f90

@@ -0,0 +1,19 @@
+program download_file
+    ! This program demonstrates using a GET request to download a file.
+    use http, only: response_type, request
+    use stdlib_io, only: open
+    implicit none
+    type(response_type) :: response
+    integer :: file
+
+    response = request('https://avatars.githubusercontent.com/u/53436240')
+
+    if (response%ok) then
+        file = open('fortran-lang.png', 'wb')
+        write(file) response%content
+        close(file)
+    else
+        error stop response%err_msg
+    end if
+
+end program download_file