ICU normalization character filter

Normalizes characters as explained here. It registers itself as the icu_normalizer character filter, which is available to all indices without any further configuration. The type of normalization can be specified with the name parameter, which accepts nfc, nfkc, and nfkc_cf (default). Set the mode parameter to decompose to convert nfc to nfd or nfkc to nfkd respectively:

Which letters are normalized can be controlled by specifying the unicode_set_filter parameter, which accepts a UnicodeSet.

Here are two examples, the default usage and a customised character filter:

 PUT icu_sample {
  "settings": {
    "index": {
      "analysis": {
        "analyzer": {
          "nfkc_cf_normalized": { 1
            "tokenizer": "icu_tokenizer",
            "char_filter": [
              "icu_normalizer"
            ]
          },
          "nfd_normalized": { 2
            "tokenizer": "icu_tokenizer",
            "char_filter": [
              "nfd_normalizer"
            ]
          }
        },
        "char_filter": {
          "nfd_normalizer": {
            "type": "icu_normalizer",
            "name": "nfc",
            "mode": "decompose"
          }
        }
      }
    }
  }
}

1. Uses the default nfkc_cf normalization.
2. Uses the customized nfd_normalizer token filter, which is set to use nfc normalization with decomposition.