This PR add docfx clean command feature.

It cleanup following directories.

1. docfx build output directory (e.g. `_site)
2. docfx metadata directory (e.g. obj, api)

These directory must be placed under docfx.json existing directory.
If it's specified to outside of docfx directory. clean operation is skipped with warning message.
And when file/directory has LinkTarget it's also skipped. Because it may pointing outside content.

In the future, It may consider supporting the --Force option.
But for now, it's only support under docfx directory files.