lua: Add support for setting global variables

This adds a global() option to the lua destination, where one can list name-value pairs, which will be injected into the lua state upon startup, shortly after the script is loaded. Currently, it supports only strings and 32-bit integers.

An usage example is included in the elasticsearch example.

It (ab)uses value-pairs to collect the global options. It would likely be better to use a GTree or hashtable instead, as we do not need the whole value-pairs infrastructure. But once the interface is agreed upon, we can easily change the code behind it.