Ensure inline markup in titles is correctly stripped

Hello,

This pull request ensure there is no inline markup such as `strong` or `em` tags in the headers' anchors. Actually, we could reduce computation time with the stripped version of the text but I think that it would be hard since the function is called *via* callbacks so user won't have the markup overriding the default ones. Moreover, I've written [a tiny benchmark](https://gist.github.com/robin850/ff2939a3d3bd61ee704c) in pure Ruby and it seems that this is more or less the same:

|            | user     | system   | total    | real         |
|------------|----------|----------|----------|--------------|
| Original   | 0.430000 | 0.020000 | 0.450000 | (  0.595127) |
| New        | 0.520000 | 0.000000 | 0.520000 | (  0.553309) |

**Note** : Please don't delete the branch of this PR ; I have still some other issues to fix. Thank you! :smiley: 

Have a nice day.