script-variables: Support LoadFromFileXXX

Based on 75a4481 from master.

Add support for script variables in LoadFromFileXXX configuration to trunk-tracking,
which can be enabled by adding "pagespeed ProcessScriptVariables on" in
the http{} block.
Also adds a helpful comment to ps_dollar().

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/549
Docs: https://github.com/pagespeed/ngx_pagespeed/wiki/Script-variables-in-LoadFromFile-configuration

Resolved conflicts:
	src/ngx_rewrite_driver_factory.h
	src/ngx_rewrite_options.cc
	test/nginx_system_test.sh
Merge pull request #765 from pagespeed/oschaaf-trunk-tracking-script-support

script-variables: Support LoadFromFileXXX