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_int_t ps_dollar(
    ngx_http_request_t *r, ngx_http_variable_value_t *v, uintptr_t data) {
```