double equal for ==highlighting==?

Hi Adam,

Any chance you could make it so your very cool extension allows users to use some kind of (yellow) text highlighting, like:

Here are ==some very important words==.
Here are `some very important words`.
Here are some very important words.

```
Here are ==some very important words==.
Here are `some very important words`.
Here are some very important words.
```

?

See: https://stackoverflow.com/questions/25104738/text-highlight-in-markdown