Automatically require plugins from the :jekyll_plugins group of the Gemfile

This allows Bundler users to easily install gem-based Jekyll plugins by simply adding a :jekyll_plugins group to their Gemfile, for example:

```ruby
group :jekyll_plugins do
  gem 'jekyll-liquid-plus'
  gem 'jekyll-stitch-plus'
end
```

This reduces the number of steps for plugin installation, encouraging gem-based plugins while working within Jekyll's existing plugin system.

Note that this change doesn't require Bundler to be installed, and gracefully rescues the load error. To the user this will be entirely transparent and it will simply improve things for those using Bundler.