add close_older option (like filebeat)

[fbaligand]

This would be great to add an option (for example 'close_older') to close handles to files that have no write activity since a few time.
Without this option, when I have numerous different files written, I have to restart logstash each day, because of that.

FileBeat has such an option :
https://github.com/elastic/beats/blob/master/filebeat/docs/configuration.asciidoc#close_older

[magnusbaeck]

Isn't this already done via the close_stale_files method?

[fbaligand]

Well, I just looked at doc, this option seems to be not documented :(

[sarkortrantor]

note that there is a bug in create_if_deleted logic where the plugin just remove the file handle from its state without releasing it.
hence it prevents the close_stale_files mechanism to achieve its purpose since the file is just no longer present in the @files hash.

I submitted #92 to address this.