Mor customizable json plugin title.

A small customization for all json plugins. New variable added in title {{HOST}}
Maybe it is time to use an array of search keys and replacements? Something like this (untested):
```php
$stuff = array(
    '{{PI}}' => GET('pi'),
    '{{TI}}' => GET('ti'),
    '{{HOST}}' => GET('h'),
);
$obj->rrd_title = str_replace(array_keys($stuff), array_values($stuff), $obj->rrd_title);
```