script-variables: Support LoadFromFileXXX

Add support for script variables in LoadFromFileXXX configuration,
which can be enabled by adding "pagespeed ProcessScriptVariables on" in
the http{} block.

Note that tests currently can't pass because of a failing unrelated test:
"start_test PageSpeed CSS loaded in fallback mode is always chunked"
I have tested this by commenting that test, but re-enabled the test before
making the pull to keep the diff clean.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/549
nit: project style is:

```
ngx_http_variable_t* var = ngx_http_add_variable(
    cf, &name, NGX_HTTP_VAR_CHANGEABLE);
```