Add option to escape non-ASCII sequences in plain strings when requested

[metthal]

Since the recent changes in YARA, non-ASCII sequences are not allowed in strings. We are trying to keep the encoding the same in yaramod but it would be nice if yaramod would be able to provide automatic escaping of non-ASCII sequences when requested. It wouldn't be necessary at all times, since users most likely don't want their rules be changing under their hands for example when yaramod is used in editors. However when the ruleset is undergoing final processing, it would be great if we were able to request escaping. Please find a way to to make this parametrized.