Allow joining lists in configuration

[lubomir]

It would be convenient if it was possible to concatenate lists in configuration files. Particularly if one file is included in another, it would be nice to be able to define common base in one file and append something in another.

Proposed syntax:

a = [1, 2, 3]

a = a + [4]

There is already a precedent for having operators in the configuration: it supports string interpolation with % operator.