Registered query: Allow to escape parameter values.

[lukas-vlcek]

We need to allow escaping input parameter values before they are applied to Mustache template. Specifically, we need to be able to escape regex special characters (like *).

This will probably lead to adding new functionality to https://github.com/searchisko/structured-content-tools

[lukas-vlcek]

In some cases we can probably use \Q ... \E regex escape sequence directly in the Mustache template. More info about this here and here.

For example we can use:

      "terms": {
        "field": "sys_tags",
        "include": {
          "pattern": "^\\Q{{query}}\\E.*",
          "flags": "CASE_INSENSITIVE|UNICODE_CASE"
        }
      }