Fixed wrong usage when invalid option was specified.
Fixed the following problem: The root directory's files are found even
if they are specified in `ignore` option.
Merge pull request #5 from yosssi/fix-ignore-option

Fixed the following problem: The root directory's files are found even if they are specified in `ignore` option.
Used filepath.Join method instead of joining path string my own.
Merge remote-tracking branch 'origin' into HEAD