Auto remove the "hexo-theme-" prefix when install a theme and support custom git origin

[Muska-Ami]

I'm a Chinese so the issues grammar in use might not correctly.

When people install a theme they usually use Git command like this to remove the prefix:

git clone https://{origin_url}/{author}/{repo}.git themes/{theme_name_without_prefix}

This way they can type "theme name" instead "hexo-theme-theme name" on the configuration file _config.yml.

And although mostly theme are publish on GitHub, but I think it's also need to support other Git remote origin and custom branch,so could you support for this? PLZ