Specify dataset done flag in regex format

Preconditions
There is a business use case when custom empty file (like _SUCCESS) is used to indicate that folder is completed. In the same time for other folder _SUCCESS file is used as indication that it is completed. These two folders belong to one dataset. And there is a necessity to use data availability dataset feature.

New Feature
Please introduce a possibility to specify dataset done flag in regex format.

Example:

<datasets>
    <dataset name="xxx-in-dataset" frequency="5" initial-instance="${xxx_data_initial_instance}" timezone="${xxx_data_timezone}">
        <uri-template>${xxx_in_dir}/${YEAR}/${MONTH}/${DAY}/${HOUR}/${MINUTE}</uri-template>
        <done-flag format="regex">_SUCCESS|_XXX</done-flag>
    </dataset>
</datasets>

Regex _SUCCESS|_XXX - indicates to look for _SUCCESS or _XXX file.