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
    '';
```
Don't want to annoy but would be nice to have MYDATABASE = { host = "...."; ... }. That is, a set of sets. No problem if you don't wish to implement it.