initial copy from github.com/cihub/seelog

actual imported commit:
https://github.com/cihub/seelog/blob/dc2005a4a4388f4719bdf4e3d1cb1d3558a1a6a9/writers_filewriter.go
initial copy from github.com/cihub/seelog

actual import:
https://github.com/cihub/seelog/blob/ff10a9b3dccefc1b2dc7406bc197ca14b380bb84/writers_filewriter_test.go
make rollig file writers reusable (public)