Add "option.updateNeeded"

Users of `gulp-changed` can now customize how _"is file changed?"_ condition is determined.

The default logic still uses last modified time comparison, but there are two more built-in comparers that ignore file timestamp and use file content instead (MD5 and SHA1 hash digests). Using content comparers is useful when synchronizing files managed by git. Timestamp of those files can change at any time (e.g. `git reset`, `git rebase`, `git checkout`, etc), but not their content.

For ultimate flexibility, users can define a custom comparer function.