Support a global spf13-vim installation and user-specific overrides

[mmikitka]

On my workstation and some remote servers, I use Vim as a user other than myself (notably as root via sudo, but sometimes other users), and I want to have the same spf13-vim configuration across users on the same system. This pull request is a first take at distinguishing global and user-specific configurations on a given system.

If the spf13-vim base configuration files (.vimrc, .vimrc.bundles, etc.) are not stored in the current user's home directory, then a global configuration is assumed. If the configuration is global, then user-specific overrides are stored in $HOME/{spf13_vim_config}.user e.g., $HOME/.vimrc.local.user, $HOME/.vimrc.before.local.user, etc.

I can write documentation after this request is approved.

[CLAassistant]

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