* Validates migration version on creation, to avoid creation of duplicated versions
* Uses `os.OpenFile` with `O_CREATE|O_EXCL` to create files to avoid file collisions
* Uses `filepath.Join` to concatenate paths, making `cleanPath()` not necessary
* Prints generated filenames
* Fixes golang-migrate#238
* Supersedes golang-migrate#250

Better for Windows OS when specify -dir as /subdir, user will see C:/subdir/0001_name.up.sql rather than /subdir/0001_name.up.sql

abs path tests fails because filepath.IsAbs() treats `/subdir` path as invalid abs path at windows when drive letter is not present

Better for Windows OS systems where `/path` can be interpreted in different ways depending on working dir

OS Windows has different interpretation of `/path`, it depends on working dir.
If working dir D:\test it interprets `/path` as `D:\path`

Linux OS has less restriction on a filepath than Windows, so path invalid in windows is perfectly valid for Linux. The only invalid dir name in Linux is one ending with null string terminator (\000)

…de/migrate into cli-create-validate-version

mege new feature