Use cache for sources_non_forked

[elliotwutingfeng]

Changes

1. plugins_config.vim checks for existence of sources_non_forked_cache folder before running call pathogen#infect(s:vim_runtime.'/sources_non_forked_cache/{}'). If sources_non_forked_cache does not exist, it will copy contents of sources_non_forked to sources_non_forked_cache.
2. update_plugins.py copies contents of sources_non_forked folder to sources_non_forked_cache folder (gitignored) if sources_non_forked_cache does not exist yet. Afterwards, it updates the contents of sources_non_forked_cache from the web. If the update fails, user would at least have contents of sources_non_forked in sources_non_forked_cache.

README.md has been updated accordingly.

Benefits

• git reset --hard and git clean -d --force is no longer necessary in the update process. Addresses #615. git pull && python update_plugins.py would be sufficient.
• After running git pull to fetch this update, existing users of vimrc can either run vim or python update_plugins.py to trigger creation of sources_non_forked_cache folder.
• vimrc maintainer can continue to push changes to sources_non_forked as per normal.

Possible issues

Is there an OS-portable equivalent for call system("cp -r " . s:vim_runtime.'/sources_non_forked' . " " . s:vim_runtime.'/sources_non_forked_cache')? It works on *nix/PowerShell/Git-For-Windows but not Windows CMD.

[amix]

I find this too risky to include, especially as I can't easily test this on a Windows machine.