Add an --all flag to :download-cancel

CHANGELOG.asciidoc

@@ -28,6 +28,7 @@ Added
   clipboard.
 - New mode `word` for `hints -> mode` which uses a dictionary and link-texts
   for hints instead of single characters.
+- New `--all` argument for `:download-cancel` to cancel all running downloads.
 
 Changed
 ~~~~~~~

doc/help/commands.asciidoc

@@ -161,8 +161,13 @@ The form `:download [url] [dest]` is deprecated, use `:download --dest [dest] [u
 
 [[download-cancel]]
 === download-cancel
+Syntax: +:download-cancel [*--all*]+
+
 Cancel the last/[count]th download.
 
+==== optional arguments
+* +*-a*+, +*--all*+: Cancel all running downloads
+
 ==== count
 The index of the download to cancel.
 

qutebrowser/browser/downloads.py

@@ -948,22 +948,30 @@ class DownloadManager(QAbstractListModel):
 
     @cmdutils.register(instance='download-manager', scope='window',
                        count='count')
-    def download_cancel(self, count=0):
+    def download_cancel(self, all_=False, count=0):
         """Cancel the last/[count]th download.
 
         Args:
+            all_: Cancel all running downloads
             count: The index of the download to cancel.
         """
-        try:
-            download = self.downloads[count - 1]
-        except IndexError:
-            self.raise_no_download(count)
-        if download.done:
-            if not count:
-                count = len(self.downloads)
-            raise cmdexc.CommandError("Download {} is already done!"
-                                      .format(count))
-        download.cancel()
+        if all_:
+            # We need to make a copy as we're indirectly mutating
+            # self.downloads here
+            for download in self.downloads[:]:
+                if not download.done:
+                    download.cancel()
+        else:
+            try:
+                download = self.downloads[count - 1]
+            except IndexError:
+                self.raise_no_download(count)
+            if download.done:
+                if not count:
+                    count = len(self.downloads)
+                raise cmdexc.CommandError("Download {} is already done!"
+                                        .format(count))
+            download.cancel()
 
     @cmdutils.register(instance='download-manager', scope='window',
                        count='count')