Prism JS plugin to display filename

[plaindocs]

Hidden away at the bottom of this page:

http://prismjs.com/plugins/show-language/

is the options of using data-language CSS attribute to display a filename for the code example.

It requires adding the plugin to Prism, and then tweaking the CSS to make the display of the filename more distinct, but would allow us to cut out quite a lot of boiler plate "put this into your .travis.yml" type text.

It does however require some fairly ugly css in the markdown such as

```
Some normal code here
```
{:  data-language=".travis.yml"}

@lislis @MariadeAnton thoughts?

[lislis]

@plaindocs I'm quite fond of code snippets saying what language or file it stems from +1
If you can live with the ugly markdown, I can live with tweaking the CSS for display 💃

[plaindocs]

Sounds good to me.

[plaindocs]

@lislis wanna take a look at this and let me know what you need from my side?

[lislis]

@plaindocs sorry, I forgot about this one.
Here you go #1209

[BanzaiMan]

This was merged.