Render components inside of buttons

[Korab-Doroci]

As the title says this simple change will allow you to put components from layout.html or another .nue file inside of buttons inside of your markdown. Fixes #277

[nobkd]

@Korab-Doroci
Can you create a test, that checks, whether this creates the expected results, please? Thanks.

[Korab-Doroci]

As mentioned before, it will only work if the svg or other component is inside a .nue file. With this change it will not work if the component is inside the layout.html.

[tipiirai]

I'm thinking of a [link] tag (or similar) for this use case. I think button should not allow any markup inside since the HTML spec only allows phrasing content