Integrate loading openCypher module procedures

Summary:
All mgp_* symbols are exported from Memgraph executable, no other
symbols should be visible.

The primary C API header, mg_procedure.h, is now part of the
installation. Also, added a shippable query module example.

Directory `query_modules` is meant to contain sources of modules we
write and ship as part of the installation. Currently, there's only an
example module, but there may be potentially more. Some modules could
only be installed as part of the enterprise release.

For Memgraph to load custom procedures, it needs to be started with a
flag pointing to a directory with compiled shared libraries implementing
those procedures.

Reviewers: mferencevic, ipaljak, llugovic, dsantl, buda

Reviewed By: mferencevic

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D2538

include/mg_procedure.syms

@@ -0,0 +1,3 @@
+{
+    mgp_*;
+};