Add line ending enum to lib

[javachaos]

I forgot to give public access to LineEnding struct, so that users could use it to define the desired line endings. E.g.

conf.set_line_ending(LineEnding::Crlf);

This commit allows users to:

use simplelog::LineEnding;

[javachaos]

This commit adds a for loop over the existing tests for each possible line ending. And asserts that the last line ending is accurate. Also, to deal with the large number of files created during test, it adds a folder named test_logs to store the generated test logs.