feat: support config line_separator

[SafeteeWoW]

Add config "line_separator" which supports the following options

input: Determine line separator by the input content. This is the default.
os: Determine line separator by the operating system
lf: Use Unix Style ("\n")
cr: Use classic Max Style ("\r")
crlf: Use Windows Style ("\r\n")

Note that the default behavior is changed.
The previous behavior is "os",
but I think "input" is a more appropriate default.

Some potential issues:

1. To avoid potential test issues by git autocrlf, I have modified .gitattributes to force all files to be checkouted in LF line separator.
2. The new line separator behavior is not the same as the old behavior. The old behavior determines the output line separator by the operating system (by std::cout in text mode). The new behavior defaults to determine by the input content's line separator. I believe the new behavior is a better default.
3. Have I added enough tests?
4. Not sure if my clang-format is setup correctly. Code format may need to be fixed