Add #include directive to .stignore (fixes #424)

Though breaks #502 in a way, as .stignore is not the only place where
stuff gets defined anymore.

Though it never was, as .stignore can be placed in each dir, but I think we
should phase that out in favor of globbing which means that we can then
have a single file, which means that we can have a UI for editing that.

Alternative would be as suggested to include a .stglobalignore which is then synced
as a normal file, but gets included by default.

Then when the UI would have two editors, a local ignore, and a global ignore.

scanner/walk_test.go

@@ -128,20 +128,20 @@ func TestIgnore(t *testing.T) {
 		*/other/test
 		**/deep
 	`)
-	patterns := parseIgnoreFile(patStr, "")
+	patterns := parseIgnoreFile(patStr, "", "")
 
 	patStr = bytes.NewBufferString(`
 		bar
 		z*
 		q[abc]x
 	`)
-	patterns = append(patterns, parseIgnoreFile(patStr, "foo")...)
+	patterns = append(patterns, parseIgnoreFile(patStr, "foo", "")...)
 
 	patStr = bytes.NewBufferString(`
 		quux
 		.*
 	`)
-	patterns = append(patterns, parseIgnoreFile(patStr, "foo/baz")...)
+	patterns = append(patterns, parseIgnoreFile(patStr, "foo/baz", "")...)
 
 	var tests = []struct {
 		f string