Simple-v-table should preform validation on the columns maps

[stumitchell]

though the documentation says the following

one element for each column in the table. Must contain :id,:header-label,:row-label-fn,:width, and :height. 

None of these are actually required, but if you leave out :row-label-fn you will get the following uncaught error.

Cannot read property 'cljs$core$IFn$_invoke$arity$1' of null

Also I don't think :header-label should be required as we often have examples with empty headers, and I don't think we ever use :height`