Because `extraPackages` and `overrides` expect functions as values it
has not been possible to perform merges. This adds suitable types for
these options that allow reasonable merging.
In particular support source files whose name start with `.` or
contain characters not allowed in the nix store, such as spaces.
Also add some test cases for `home.file`.
Added utilities to generate export statements and definitions for zsh scripts.
Currently, there is only lib.shell which generates export statements in bash
syntax. However, this does not allow to generate export statements for zsh
arrays (syntax: NAME=(elem1 elem2 ...) ), which would be the natural
representation of lists in the nix language.
