Initial support of templates and so on (#71)

* Initial syntax is the following:

    * $template{{ some JS code here }} - simple template based on JS
      code inside. It works the same as lambda functions, but calculates
      only once.
    * aw_count(regex) - keys count found for given regex
    * aw_keys(regex, [separator]) - keys found for given regex and
      joined by using given separator
    * aw_names(regex, [separator]) - key names found for given regex and
      joined by using given separator (the same as previous but w\o $)

  The template and function syntax may be changed before release.

* replace `foreach` to `for (auto foo : bar)` and update CONTRIBUTING.md
  accordingly

sources/awesome-widget/plugin/awdataaggregator.cpp

@@ -131,7 +131,7 @@ QPixmap AWDataAggregator::tooltipImage()
     toolTipScene->clear();
     QPen pen;
     bool down = false;
-    foreach (QString key, requiredKeys) {
+    for (auto key : requiredKeys) {
         // create frame
         float normX = 100.0 / static_cast<float>(data[key].count());
         float normY = 100.0 / (1.5 * boundaries[key]);