Fixed issue with idea-multimarkdown.

- JavaFX does not support lookbehind
 - (?) It also appears that named regex groups are also unsupported for both mermaid and javafx

Update:
 - Fixed an issue where setLogLevel did not properly handle 'named' log levels
 - Backwards compatibility should be preserved, any/all %%{...}%% directives will be correctly processed by the grammar and properly ignored for any/all graph types that do not support them.

 - Multiline directives will render an error (as they should) if they are not accounted for in the .jison grammar

src/logger.js

@@ -16,7 +16,13 @@ export const logger = {
   fatal: () => {}
 };
 
-export const setLogLevel = function(level) {
+export const setLogLevel = function(level = 'fatal') {
+  if (isNaN(level)) {
+    level = level.toLowerCase();
+    if (LEVELS[level] !== undefined) {
+      level = LEVELS[level];
+    }
+  }
   logger.trace = () => {};
   logger.debug = () => {};
   logger.info = () => {};