Transform list of unicode chars in templater

Since OTP 17 strings read from a file are by default parsed as a list of
unicode characters.
If such a list contains a non-ascii character (unicode > 255) then a
call to erlang:list_to_binary/1 fails.
This commit converts unicode list of chars to binary via unicode module
if ran against OTP >= 17.
I believe that R16 and 17 would need a different escaping. I may ask a question on erlang-questions next week.
Well, after thinking this through, maybe it would be a better idea to simply parse string vars from files as a byte-after-byte list, i.e. not allowing Erlang to interpret unicode bytes combinations as a unicode character.
``file:consult/1`` can be tricked to do so, but I think the only option is to set the encoding in the file's content (vide docs of the ``epp`` module), but AFAICS the implementation of ``consult`` in ``rebar_templater`` does exactly that.
What do you think? Happy Holidays :)