Add Multi Backend Docs and Sample Configuration

This pr adds a bit of documentation on how to configure and use the multi pgsql backend. It adds some markdown in the docs section as well as a sample config/style file and a lua file for tag transformations.

In terms of code changes I added a bit to the output of the --help -v output to describe and note the multi backend exists. I also added a fix to the multi backend options usage for specifying the name of the tables. It seemed unintuitive to me that when you specify a name to the table it would still get the table prefix. If you want a prefix put it in the config for the table. It seems like the prefix is useful in the  normal pgsql backend where you dont have full control over the names of the tables and didn't apply to the multi backend.

i've also open issue #227 noting the lack of documentation and samples for this. this pr addresses it
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