Add zooming with Ctrl-mousewheel.

Closes #51.

doc/help/settings.asciidoc

@@ -83,6 +83,7 @@
 |<<input-spatial-navigation,spatial-navigation>>|Enables or disables the Spatial Navigation feature
 |<<input-links-included-in-focus-chain,links-included-in-focus-chain>>|Whether hyperlinks should be included in the keyboard focus chain.
 |<<input-rocker-gestures,rocker-gestures>>|Whether to enable Opera-like mouse rocker gestures. This disables the context menu.
+|<<input-mouse-zoom-divider,mouse-zoom-divider>>|How much to divide the mouse wheel movements to translate them into zoom increments.
 |==============
 
 .Quick reference for section ``tabs''
@@ -785,6 +786,12 @@ Valid values:
 
 Default: +pass:[false]+
 
+[[input-mouse-zoom-divider]]
+=== mouse-zoom-divider
+How much to divide the mouse wheel movements to translate them into zoom increments.
+
+Default: +pass:[512]+
+
 == tabs
 Configuration of the tab bar.
 

qutebrowser/browser/webview.py

@@ -527,3 +527,23 @@ class WebView(QWebView):
         menu = self.page().createStandardContextMenu()
         self.shutting_down.connect(menu.close)
         menu.exec_(e.globalPos())
+
+    def wheelEvent(self, e):
+        """Zoom on Ctrl-Mousewheel.
+
+        Args:
+            e: The QWheelEvent.
+        """
+        if e.modifiers() & Qt.ControlModifier:
+            e.accept()
+            divider = config.get('input', 'mouse-zoom-divider')
+            factor = self.zoomFactor() + e.angleDelta().y() / divider
+            if factor < 0:
+                return
+            perc = int(100 * factor)
+            message.info(self.win_id, "Zoom level: {}%".format(perc))
+            self._zoom.fuzzyval = perc
+            self.setZoomFactor(factor)
+            self._default_zoom_changed = True
+        else:
+            super().wheelEvent(e)

qutebrowser/config/configdata.py

@@ -392,6 +392,11 @@ DATA = collections.OrderedDict([
          SettingValue(typ.Bool(), 'false'),
          "Whether to enable Opera-like mouse rocker gestures. This disables "
          "the context menu."),
+
+        ('mouse-zoom-divider',
+         SettingValue(typ.Int(minval=1), '512'),
+         "How much to divide the mouse wheel movements to translate them "
+         "into zoom increments."),
     )),
 
     ('tabs', sect.KeyValue(