Change handler to use a defined character for line-break replacement

This patch is the first in a series aimed at making it easier for DW to allow
plugins to modify the standard handling of line-breaks.

Currently the handler "eats" line-break characters replacing them with a space.
This patch changes the replacement character to a define, DOKU_PARSER_EOL and
updates all the test cases to use the new define.

DOKU_PARSER_EOL is left as a space.

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