Add preprocess option to edoc_opts
Add preprocess option to edoc_opts

This prevents edoc from getting syntax errors when using macros. At this
stage, Riak CS still has several edoc issues, which will get fixed in a
subsequent commit.

dragonfax/riak_cs,sdebnath/riak_cs,sdebnath/riak_cs,GabrielNicolasAvellaneda/riak_cs,laurenrother/riak_cs,GabrielNicolasAvellaneda/riak_cs,basho/riak_cs,sdebnath/riak_cs,laurenrother/riak_cs,laurenrother/riak_cs,basho/riak_cs,dragonfax/riak_cs,basho/riak_cs,dragonfax/riak_cs,dragonfax/riak_cs,GabrielNicolasAvellaneda/riak_cs,laurenrother/riak_cs,GabrielNicolasAvellaneda/riak_cs,basho/riak_cs,yangchengjian/riak_cs,sdebnath/riak_cs,yangchengjian/riak_cs,yangchengjian/riak_cs,yangchengjian/riak_cs