New option to filter the plugins to run at startup

[zepouet]

Hello,

I just add a new option to filter plugins to enable at startup.

In the future, we will have a lot of plugins into all.go and into config.toml.
A solution would be to have many differents config files.
Another one could be to use this option to filter the plugins.

telegraf --config telegraf.toml -filter mysql:redis

Maybe I am wrong but If I want to use the same config file and activate only one plugin, the filter option could be nice. Else I must duplicate config file and delete other plugin to disable them.
The config file share common configuration so it is nice to avoid to duplicate it.

Thanks for reading this PR.

Nicolas

[sparrc]

@zepouet thanks! this look like a useful addition,

please rebase your changes with influxdb/telegraf and fix the gofmt issues