Add ability to exclude identifiers fixes #15

testdata/scripts/imports.txt

@@ -3,6 +3,7 @@ exec ./main
 cmp stdout main.stdout
 
 ! binsubstr main$exe 'ImportedVar' 'ImportedConst' 'ImportedFunc' 'ImportedType' 'main.go' 'imported.go'
+binsubstr main$exe 'ImportedAPI'
 
 [short] stop # checking that the build is reproducible is slow