Fix some keybindings

* The `ctrl+a` and `ctrl+e` had a strange behavior when the mark was set, after checking I found out that `extend` was `false` where I guess it was supposed to be `true`.

* In emacs, `alt+f` and `alt+b` stop at underscores, so for example if we have the cursor at the begining of a line with 
```
foo_bar
```
and press alt+f, the cursor should be before the underscore, however with the current bindings it ended up at the end of the word.