Split lines in lexer only by \r\n, \r and \n

Split lines in lexer only by \r\n, \r and \n

Python str.splitlines() splits by more characters[1], which, however,
causes problems when keeping these special characters in processed
templates is desirable, i.e. these bug reports: #769, #952, #1313.

The keep_trailing_newlines logic is reworked because splitlines()
removes them already (so they had to be added), while re.split doesn't
so they have to be removed.

[1] https://docs.python.org/3/library/stdtypes.html#str.splitlines