Change the way @since work

[KittyGiraudel]

Currently @since is basically a message annotation. You give it a message, it gets displayed. I want to change that so it has 2 parameters: a version or whatever, and optionally a message.

/**
 * @since 1.3.37
 */

Description starts at first space with an optional hyphen, as usual.

/**
 * @since 1.3.37 - now makes a good use of `inspect()`
 */

This would allow a more robust view.

[KittyGiraudel]

Could you review it @FWeinb: https://github.com/SassDoc/sassdoc/tree/improved-since-annotation?