Pragma once preprocessor directive support added.

Support for the non-standard but widely supported preprocessor directive
#pragma once. The script will remove all the pragmas from the included
content, except the root file.
Angled and quoted includes now differ.

File included with quotes is searched in the 'parent' file directory
first. This is complying to most compiler implementations.
Datetime placeholders in prologue file

Prologue file can now contain Python datetime strftime placeholders to
add amalgamation time to resulting file prologue.