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/awkeysaggregator.cpp

@@ -102,7 +102,7 @@ QString AWKeysAggregator::formater(const QVariant &data,
     case TimeCustom:
         output = m_customTime;
         [&output, loc, this](const QDateTime dt) {
-            foreach (QString key, timeKeys)
+            for (auto key : timeKeys)
                 output.replace(QString("$%1").arg(key), loc.toString(dt, key));
         }(data.toDateTime());
         break;