Support placeholders with #external declarations

This adds support for declaring predicates as placeholders through the
“#external” directive in the input language of clingo.

Placeholders are not subject to completion. This prevents predicates
that represent instance-specific facts from being assumed as universally
false by default negation when translating an encoding.

This stretches clingo’s usual syntax a bit to make the implementation
lightweight. In order to declare a predicate with a specific arity as a
placeholder, the following statement needs to be added to the program:

    #external <predicate name>(<arity>).

Multiple unit tests cover cases where placeholders are used or not as
well as a more complex graph coloring example.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## (unreleased)
 
+### Features
+
+* support for declaring placeholders with the `#external` directive
+
 ### Internal
 
 * drops Boost dependency in favor of the header-only command-line option library [cxxopts](https://github.com/jarro2783/cxxopts)