Fix #142: Firefox opening multiple windows

Try to open new tab and not window.
Fix test case fail.

buku.py

@@ -2239,7 +2239,7 @@ def browse(url):
     os.dup2(fd, 2)
     os.dup2(fd, 1)
     try:
-        webbrowser.open(url)
+        webbrowser.open(url, new=2)
     except Exception as e:
         logerr('browse(): %s', e)
     finally:

tests/test_buku.py

@@ -235,7 +235,7 @@ def test_browse(url, opened_url):
     with mock.patch('buku.webbrowser') as m_webbrowser:
         import buku
         buku.browse(url)
-        m_webbrowser.open.assert_called_once_with(opened_url)
+        m_webbrowser.open.assert_called_once_with(opened_url, new=2)
 
 
 @only_python_3_5