Refactor completion

[tompng]

(TODO: wait for reline_0.5.0.pre)

Fixes #611 and #621

Split logic and state of autocompletion and tab completion

Before

Reline has tab complete and autocomplete.
Tab complete uses

@completion_state = NORMAL | COMPLETION | MENU | MENU_WITH_PERFECT_MATCH | PERFECT_MATCH

Autocomplete uses

@completion_state = NORMAL | JOURNEY
@completion_journey_data
retrieve_completion_block(not stored to ivar, calling every time, causing dialog disappearing bug)

It's a separate feature but uses same instance variable @completion_state that making hard to fix some bugs.

After

Tab complete uses

@completion_state = NORMAL | COMPLETION | MENU | MENU_WITH_PERFECT_MATCH | PERFECT_MATCH

Autocomplete uses

@completion_journey_state = CompletionJourneyState.new(line_index, pre, target, post, list, pointer) | nil

State is separated. Logic is also separated. Now we can remove redundant proc calls and fix #611 #621

completion_journey_data

completion_journey_data is an API for dialog proc, defined in DialogProcScope. We need to keep compatibility.
It was an instance variable but changed to a method that calculates from @completion_journey_state.