CapPaste: add method to find if a paste is suitable for the backend

Basic support in pastoob added (there is no way yet to change the
default options).
Backends support the "public/private" requirement.

weboob/backends/pastealacon/backend.py

@@ -43,6 +43,11 @@ class PastealaconBackend(BaseBackend, ICapPaste):
     def new_paste(self, *args, **kwargs):
         return PastealaconPaste(*args, **kwargs)
 
+    def can_post(self, public=None):
+        if public is False:
+            return 0
+        return 1
+
     def get_paste(self, _id):
         with self.browser:
             return self.browser.get_paste(_id)

weboob/backends/pastealacon/test.py

@@ -79,3 +79,7 @@ class PastealaconTest(BackendTest):
         # same even with correct domain (IDs are numeric)
         assert self.backend.get_paste('http://pastealacon.com/nJG9ZFG8') is None
         assert self.backend.get_paste('nJG9ZFG8') is None
+
+    def test_can_post(self):
+        assert 0 == self.backend.can_post(public=False)
+        assert 1 == self.backend.can_post(public=True)