Add support for extra_css and extra_javascript in all the themes

[jimporter]

I noticed that most of the themes don't actually support extra_css. I think I saw you mention that these themes would eventually be removed from mkdocs core and be turned into add-ons or something, but in the meantime, I think it makes sense to maintain feature parity across the themes.

I'm not sure if you'd like tests for this, but if you do, just let me know and I'll try to figure something out.

[d0ugal]

This change looks good to me, I think a test would be useful to verify it is working with one of the themes. If nothing else it'll stop us getting into this situation again.

[jimporter]

I like tests! What's the best way to write a test for this?

[dnrce]

@jimporter I just saw this after opening #135. You may wish to consider the same change from

{% for path in extra_css %}<link href="{{ base_url }}/{{ path }}" rel="stylesheet">{% endfor %}

to

{%- for path in extra_css %}
<link href="{{ base_url }}/{{ path }}" rel="stylesheet">
{%- endfor %}

(with appropriate leading whitespace), which generates prettier HTML.

[jimporter]

@zoombody Noted. If your PR lands, I'll be sure to update this one.

Still not sure where tests for this should go, though...

[dnrce]

Also, since dc84d21 the use of full URLs is allowed in extra_* so the {{ base_url }}/ piece should be dropped.

[jimporter]

I updated and noticed that 7167406 totally broke the custom themes, since config isn't a variable for the templates anymore...

I'll file a new PR for that.

[Keats]

Looks good, could you add the extra_javascript that are missing as well ? (also +1 for @zoombody use of {%-)

[d0ugal]

I think practicality wins over purity here, we don't need a test. I'll give this a manual test next week.

[jimporter]

Ok, I updated this to include extra_javascript and to do the multi-line thing.

[d0ugal]

Sorry for the tardy followup.