Add {encoder,output}_options to ES plugin

I'd like to set the `flush_interval` option for `ElasticSearchOutput`.

However I don't want to keep modifying these plugins every time I need to
use a new option. So I'm exposing the ability to set arbitrary options using
a hash. Some futzing is required to ensure that the TOML file contains
entries of the right type.

If this is successful then I anticipate doing the same for the other
plugins. We'd probably want to keep separate `host` and `port` params,
because they are always used and some have sensible defaults like binding to
loopback. But everything else could become a hash - possibly even existing
hardcoded options.
Release 0.3.0
Merge pull request #4 from alphagov/es_arbitrary_options

Add {encoder,output}_options to ES plugin