Install, Remove and Update with user@plugin format

[bharatvaj]

Fixes #1

In the discussion, your proposed solution was to use user-plugin/ format inside the plugin folder.

The problem with that approach was that user accounts can have - in them. A plugin such as vim-scripts/Ada-Bundle would have end up as vim-scripts-Ada-Bundle/ making it impossible to differentiate between the user and plugin in uninstall step.

@ is a good candidate for splitting because user accounts cannot use @, atleast in github. Since https://user:pass@domain.com is an auth mechanism, hopefully no one allow it as username.

I tried to make the changes as minimal as possible, so there is a small unnecessary change in how dir=$1 is set.

Tested in,

• alpine ash
• Arch bash
• Windows ash