Allow the inclusion of files, from FILE_CONFIG

[joseluis]

Allow including files, that are part of the configuration, from the main project file config (.weberarc by default), and keep nesting from the included parts, respecting Nesting_max.

IDEA: Use uppercase names surrounded by underscores to make them stand out.

• _INCLUDE_
• _REQUIRE_

FEATURES:

• _INCLUDE_ should support specifying a directory (or globbing (bash 4)) somewhere/**/**.conf
• Model it like embedding templates.
• In principle, both operations should be able to be nested too.
• Relative paths are relative to the script in which include/require operation is written. But absolute paths can be used too.