Add parameter to Pgsql module for easy reuse

[eMerzh]

Hi,
i've wanted to create my custom munin plugin for an application i've made.
This plugin can be based on Munin::Plugin::Pgsql module since it use postgresql. But i want it to be under it's own category and not under the Postgresql category.
So i've "patched" the module to permit that it's really simple and i think it should be applied so everyone can use it :)

Thanks

[eMerzh]

Thanks for applying my patch... this can be closed

[snide]

Done in cf16b17c62cb228d580fb13719a18c57e4e8891b.