home-environment: check files for '.' prefix
home-environment: check files for '.' prefix

Nix does not allow files whose name start with a '.' in the Nix store.
This commit makes a not of this fact in the `home.file.source` option
and also adds an assertion verifying that no such file is given.

Closes #4

rycee/home-manager,rycee/home-manager,rycee/home-manager