Add a greasemonkey.init()

This also creates the greasemonkey directory if it doesn't exist yet, for
discoverability.

qutebrowser/app.py

@@ -492,8 +492,7 @@ def _init_modules(args, crash_handler):
     objreg.register('cache', diskcache)
 
     log.init.debug("Initializing Greasemonkey...")
-    gm_manager = greasemonkey.GreasemonkeyManager()
-    objreg.register('greasemonkey', gm_manager)
+    greasemonkey.init()
 
     log.init.debug("Misc initialization...")
     macros.init()

qutebrowser/browser/greasemonkey.py

@@ -29,7 +29,7 @@ import glob
 import attr
 from PyQt5.QtCore import pyqtSignal, QObject, QUrl
 
-from qutebrowser.utils import log, standarddir, jinja
+from qutebrowser.utils import log, standarddir, jinja, objreg
 from qutebrowser.commands import cmdutils
 
 
@@ -209,3 +209,14 @@ class GreasemonkeyManager(QObject):
     def all_scripts(self):
         """Return all scripts found in the configured script directory."""
         return self._run_start + self._run_end + self._run_idle
+
+
+def init():
+    """Initialize Greasemonkey support."""
+    gm_manager = GreasemonkeyManager()
+    objreg.register('greasemonkey', gm_manager)
+
+    try:
+        os.mkdir(_scripts_dir())
+    except FileExistsError:
+        pass