Add :crossovers support

[magomimmo]

It could happen that a CLJS lib includes a component implemented as a pure CLJ/CLJS code. for example, Enfocus is a DOM manipulation library inspired by Enlive which indirectly uses, through a CLJ macros' source file, the https://github.com/ckirkendall/enfocus/blob/master/src/clj/enfocus/enlive/syntax.clj CLJ source file. This file is written in pure CLJ/CLJS and has to be used, through the above macros' file, in the CLJS codebase as well. This is a case in which a CLJS lib needs to use the :crossovers option from the cljsbuild plugin.

Think if it's possible to add the management of the :crossovers option in the cljs-start.