Add minify support

[areille]

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Closes #542

[areille]

Gone to WIP status because of a problem found with minify-html.

It appears to turn :
<link href="http:&#x2F;&#x2F;areille.com&#x2F;style.css" rel="stylesheet">
into :
<link href=http:%%areille.com%style.css rel=stylesheet>

[areille]

@Keats this is ready for review now.

[Keats]

I think we should move the minification to write_content so we can't forget it and no need to repeat ourselves everywhere

[areille]

I moved the minify() call inside write_content(). Hope that it is ok now !

[Keats]

Seems good, can you add a couple of tests for the minified output?

[areille]

I added a test for index.html. Please let me know if this is not enough and I will add some tests for other files.

EDIT : it appears to break other tests, will correct that

[Keats]

Thanks! I'll bump the pinned version myself for the build to pass