Changed the default line ending in vcf.Writer() to '\n'.

Currently, the default is '\r\n'. Most vcf files in public repositories use '\n' line endings. And vcf.Writer() writes the meta data directly with the original line ending '\n', but changes the record line ending to '\r\n' by default, which is quite confusing (and may cause compatibility issues). 

This is just a quick fix. A better way to approach this is to check the line ending in the orignal vcf file served as the template, and use it as default. I am not sure whether this is worth it though.