interfaces for completion
file completion
remove Guardfile
get completer on on() and add complete() funcref to an object

implement complete function is TODO
known option completion for long and short options
follow the chagen of interface in completion function
add '=' if the option has value and refactor a bit
change interface of completion function

now it takes 3 arguments rather than 1.
argument 2 is commandline and argument 3 is cursor position.
add spec for completion
fix short option detection
rename confusing argument name
fix indentation
about completion feature
Merge branch 'add/completion'