Improved conf for evil-leader.

- Add some extra bindings for helm.
- Fix bindings for expand-region since expand-region is autoloaded and
`(featurep 'expand-region)` will return nil. So I use
`(require 'expand-region nil 'noerror)` instead. This fix #67.
Use `eval-after-load` instead of `require` or `featurep` for evil-leader.