Update filelog.go

分割日志时，分割文件名不包含日期。
Update filelog.go

fix Misspelling in line 143 "w.Filename ==> w.filename"
add judgement of the kind of ratate

add judgement of the kind of ratate.
If it is daylyRatate the date string used in oldfilename should be yesterday.
If not ,use today.
Merge pull request #3 from cfxks1989/master

change the format of filename
Merge branch 'master' of https://github.com/alecthomas/log4go