Support include directory for configuration files

Hi!
As you did with Telegraf, it would be great to add the possibility for using an include directory for configuration. This would enable socket UDF modules individually packaged as Docker containers for example.

Something like telegraf implementation sounds perfect for this use.

kapacitor.d/module1.conf would look like:
`[udf.functions.module1]`
`socket = "/tmp/module1.sock"`
`timeout = "10s"`

Thanks a lot!

Start by reviewing how Kapacitor currently loads configuration and compare it with Telegraf's include-directory implementation. Determine how files such as kapacitor.d/module1.conf should be discovered and merged, then verify that individually configured socket UDF modules are loaded with their socket and timeout settings.

Written by the indexing model from the issue text.