Adding logtext formatter
Made file name/line number part of logrus core
Saving off entry string for use in panic

as the reader is not seekable and without this the panic string is
always empty :scream_cat:
Switching to io.MultiWriter for clarity
Adding tests for PanicLevel behavior, plus changing behavior!  :scream_cat:
Making panic level assertions more helpful
Merge branch 'addin-file-line-stacktrace' of github.com:dogenzaka/logrus into addin-file-line-stacktrace