Simple nixos module to enable configuration of freetds

The freetds library relies on a few environment variables and a `freetds.conf` file for setting up connections.

This modules adds an easy way to write entries to `freetds.conf` and to setup the environment.

Example:

```
environment.freetds.MYDATABASE = 
    ''
    host = 10.0.2.100
    port = 1435
    tds version = 7.2
    '';
```
Yeah I was unsure of the correct spot myself. It certainly wasn't a `service` so I decided to mimic `environment.wvdial`.

If you think `programs` is the better area I'll tweak, but since this is configuration for how a _library_ behaves that didn't quite feel write either.