[Vim] Manage plug-ins with spore

I've always used the Pathogen + git submodules method of managing Vim
plug-ins. It worked okay until I had to clone my setup onto another
machine. There were a couple of issues:

- I had accidentally made merge commits in several of the submodule
  repositories, so I had to remove those before being able to pull down
  certain plug-ins.
- There were a couple repositories of my own that I could not clone
  without uploading a key to GitHub or changing the URL in my
  .gitmodules file.

Those issues along with various other inconveniences related to managing
submodules prompted me to look for another solution. I first tried out
the popular Vundle. Overall, I thought it was great, however it tripled
Vim's startup time over my previous setup, which is not acceptable. I
looked into NeoBundle, knowing that it was similar but could lazy-load
plug-ins. By the time I finished reading the documentation, it was clear
that NeoBundle was just overkill. My needs are too simple for this
excessively complicated plug-in. I eventually came across vim-plug,
which advertises itself as something "in between Pathogen and Vundle".
Indeed it was faster than Vundle (and could install plug-ins in
parallel), but it still doubled my startup time under Pathogen.

I then turned my attention to external managers including Pandemic,
Epidemic, and Infect. All of these work with Pathogen, as the names
suggest, but live as external scripts that you use from the command
line. All of them fall short in some way from meeting my requirements:

- Works with my existing Pathogen setup
- Does not introduce a dependency on Ruby or Python
- Maintains its list of plug-ins in a file that can be versioned
  (preferably in my vimrc)
- Optimized for use with plug-ins hosted on GitHub

This first draft of spore meets these requirements. It works by reading
a specially formatted list of repositories in my vimrc. It will even run
build commands for plug-ins such as YouCompleteMe that require
compilation.
[Vim] Slim down customizations