main: compute function & bb layout

so bb can be associated with function in output.
only captures BBs that have a rule match,
otherwise, there might be too much data captured.
closes #130.

scripts/show-capabilities-by-function.py

@@ -174,6 +174,7 @@ def main(argv=None):
     meta = capa.main.collect_metadata(argv, args.sample, args.rules, extractor)
     capabilities, counts = capa.main.find_capabilities(rules, extractor)
     meta["analysis"].update(counts)
+    meta["analysis"]["layout"] = capa.main.compute_layout(rules, extractor, capabilities)
 
     if capa.main.has_file_limitation(rules, capabilities):
         # bail if capa encountered file limitation e.g. a packed binary