Handle variables

[l0kod]

Configuration files could be dynamically generated, especially is JSON, but it could lead to common issues such as injection attacks if not properly escaped, which might be tricky for file paths.

Users should also be able to easily customize a configuration file according to their environment and context (e.g. $HOME, $TMPDIR).

All variables should be explicitly declared in the configuration file using them, but they may be marked as empty/ok-if-unused.

Variables declared in configuration files should have a scope: local (default) or global (for shared variables).

The ability to declare and use variables may also be required for #20.