Consider allowing vals for tag-fn, similar to transit-clj

[dmarjenburgh]

In transit-clj, the write-handler function allows for the tag-fn (and other fns) to be values. From the doc:

If a non-fn is passed as an argument, implemented
handler method returns the value unaltered.
https://github.com/cognitect/transit-clj/blob/master/src/cognitect/transit.clj#L73-L76

In transit-cljs you must always pass a function. Making write-handler behave the same in clj and cljs
makes sharing code for write-handler fns in cljc files a bit easier.

[swannodette]

Fixed c165b6f