Handle boolean sorthand for filters (#2977)

* Handle boolean sorthand for filters

[closes #2966]

* Commit the yarn lock

Author: ForbesLindesay

.gitignore

@@ -12,4 +12,3 @@ npm-debug.log
 .release.json
 lerna-debug.log
 package-lock.json
-yarn.lock

.travis.yml

@@ -7,7 +7,7 @@ node_js:
 # Use faster Docker architecture on Travis.
 sudo: false
 
-script:        npm test
+script: yarn test
 # TODO: coverage
 # after_success: npm run coveralls
 

packages/pug-filters/lib/handle-filters.js

@@ -94,7 +94,7 @@ function getAttributes(node, options) {
   var attrs = {};
   node.attrs.forEach(function (attr) {
     try {
-      attrs[attr.name] = constantinople.toConstant(attr.val);
+      attrs[attr.name] = attr.val === true ? true : constantinople.toConstant(attr.val);
     } catch (ex) {
       if (/not constant/.test(ex.message)) {
         throw error('FILTER_OPTION_NOT_CONSTANT', ex.message + ' All filters are rendered compile-time so filter options must be constants.', node);