[RFC] Add helper script for porting vim patches.

As a follow-up from @justinmk's comment in https://github.com/neovim/neovim.github.io/pull/75#issuecomment-63788722, here's a simple script that helps to automate the workflow of porting a Vim patch.

```
$  ./scripts/vim-patch.sh
Helper script for porting Vim patches. For more information,
see https://github.com/neovim/neovim/wiki/Merging-patches-from-upstream-vim.

Usage: ./scripts/vim-patch.sh vim-version
vim-version must be in format '7.4.xxx'.

Set VIM_SOURCE_DIR to change where Vim's sources are stored.
The default is '/home/florian/Projects/neovim/build/vim'.
$ export VIM_SOURCE_DIR=$(pwd)/../vim
$ ./scripts/vim-patch.sh 7.4.115
Updating Vim sources in '/home/florian/Projects/neovim/../vim'.
pulling from https://code.google.com/p/vim/
searching for changes
adding changesets
adding manifests
adding file changes
added 23 changesets with 58 changes to 33 files
33 files updated, 0 files merged, 0 files removed, 0 files unresolved
Using Vim tag 'v7-4-115'.

Creating Neovim branch 'vim-7.4.115'.
Switched to a new branch 'vim-7.4.115'

Saving patch to '/home/florian/Projects/neovim/vim-7.4.115.patch'.
Saving full commit details to '/home/florian/Projects/neovim/vim-7.4.115.commit'.

Creating empty Neovim commit with correct commit message.
[vim-7.4.115 539350d] vim-patch:7.4.115

Proceed to port the patch and stage your changes ('git add ...').
Then use 'git commit --amend' to commit.
Push your changes with 'git push origin vim-7.4.115' and create a
pull request called '[RFC] vim-patch:7.4.115'.

See https://github.com/neovim/neovim/wiki/Merging-patches-from-upstream-vim
for more information.
```