add directive 'mruby_set' and 'mruby_set_inline'

These directives assign a result of mruby script to a variable.

```nginx
mruby_set_inline $res1 "ARGV[0] + ', ' + ARGV[1]" "Hello" "World";
mruby_set        $res2 "/etc/nginx/script/hello.mrb";
```

BTW, These directives require [ngx_devel_kit](https://github.com/simpl/ngx_devel_kit).