Support for writing multiline indent value

## Add support from writing multiline indent values By default newline chars are escaped on write operation. This cause problems with some parsers with no support for escaped (i.e python ConfigParser), those parser returns the values with their escaped values and not the original value. This is orthogonal to the escaping policies and need to be treated in a specific way. ### Changes * Add a WriteOption `indent_multiline_value` , `false` by default, which allow for supporting writing multiline values that spans across lines. * Modified the `escape_str` function to specifically handle indentation of spanned lines. * Added tests Thanks for this library !