Allow keywords in style maps

[shhivam]

I made a copy of PR#140 since that PR was no longer active

From PR#140 of Paulus Esterhazy pesterhazy@gmail.com:

Reagent allows keywords as CSS attribute values such as
{:style {:color :red}}

Hiccup already supports this for top-level DOM node attributes. This commit
extends this feature to style maps.

[shhivam]

Thanks @weavejester for the quick merge! This was my very first open source contribution (although very very small)

[weavejester]

Well done 😃