Configure function registration for SQLite and RRD plugin

Currently it's not possible to use the SQLite and RRD plugin at the same time, since it registers two functions
* db()
* series()
to all items having the SQLite or RRD plugin enabled. To make it possible to use both plugins at the same time, you can now use the `register` configuration which configures a map for the function registration of the mentioned functions above.

The default for this would be `register = _single : db | _series : series`, which can be overwritten in one of the plugins to make it possible to the let the other plugin register too.

Keep in mind that the visu plugin is also using the `series()` function, which should be registered by one of the plugin to keep the visu plugin running!

This patch is more a preparation to make it possible to decide which plugin should be used to provide historical data for items (sqlite plugin or rrd plugin when you want running both of them). The next step I want to implement is, to let the `dblog` plugin provide the data too.

This way you're flexible as much as possible since you can decide which plugin you want to use and which plugin you want to provide the historical data.