Deoplete needs `get_complete_position` method and it has a different
signature. It already fetches the input string and attempts to detect
the position with `\k*` regexp patterns.
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 `''`.
