Use `--ff --ff-only` instead of `--rebase` with `git pull`

Given a dirty worktree (with changed files), `git pull --ff --ff-only`
behaves better that `git pull --rebase` when updating plugins.

Fixes: https://github.com/Shougo/neobundle.vim/issues/305
Add option `g:neobundle#types#git#pull_command`
Improve documentation
Merge pull request #306 from blueyed/git-use-pull-ff-ffonly

Use `--ff --ff-only` instead of `--rebase` with `git pull`