skip collecting files inside dot directory by default

[Songmu]

follow on #63

Currently, only dot files are skipped, files inside dot directory are not skipped.

I do not know if this is the intended behavior, but I implement that skips the collection of files inside dot directory with using filepath.Skipdir.

Also, I create new Option ziptree.IncludeDotfiles for collecting dotfiles and utilize it in statik.go for adding flag "-dotfiles" to zip dotfiles.