Allow escaping all non-ASCII characters in .properties

[rimas-kudelis]

It would be nice if I could choose to have all non-ASCII characters escaped in Java .properties files. It could be done as an option, or by default, or as a new format, whichever suits you better.
Rationale:

1. There are a lot of languages which don't fit into ISO-8859-1, but use some characters present in it. They currently get a mish-mash of escaped and non-escaped accented symbols. It's just ugly.
2. At least the project that I'm working on has historically used the escaped form of non-ASCII symbols. Now, each time after tx pull, I have to go through all the noise to undo all this "unescaping" done by tx, and find the real changes. It's boring and time-consuming, and counter-productive.
3. Last and the least, some tools freak out when stumbling upon non-UTF-8 characters. For example, this is currently happening with gitg. Surely, it's theirs bug, not yours, but I still wanted to mention it.