Case-insensitive APIs? Say it isn't so!

According to [lib/logstash/filters/grep.rb#53](https://github.com/logstash/logstash/blob/master/lib/logstash/filters/grep.rb#L53) and my understanding of [Regexp::new](http://ruby-doc.org/core-2.0/Regexp.html#method-c-new), I don't think so.

I see the workarounds as:

1. Submit an upstream issue/PR to add a `case_insensitive` option to the `grep` filter; and/or
2. Add a few some additional mutations to make it easier to work with the field. Something like...

    mutate {
        type => "iis_tradingapi"
        add_field => [ "cs_uri_stem_lower", "%{cs_uri_stem}" ]
        lowercase => "cs_uri_stem_lower"
    }

    # your existing generated grep rules, but using `cs_uri_stem_lower` instead of `cs_uri_stem`

    mutate {
        type => "iis_tradingapi"
        remove => [ "cs_uri_stem_lower" ]
    }