Add input_pattern setting for deoplete

This option is used to determine if `min_pattern_length` is ignored.
In usual, it does not start completion when the matched input string is
shorter than `min_pattern_length`. But when the string matches
`input_pattern`, it starts completion even when ths string is `''`.

rplugin/python3/deoplete/sources/ale.py

@@ -24,6 +24,7 @@ class Source(Base):
         self.rank = 1000
         self.is_bytepos = True
         self.min_pattern_length = 1
+        self.input_pattern = r'(\.|::|->)\w*$'
 
     # Returns an integer for the start position, as with omnifunc.
     def get_completion_position(self):