Custom outputters

This should add support for writing and using custom outputters. One could for example write bash parsable outputter state (csv?), or xml...

In salt, outputters code could be ran on master (salt --out=... cmd.run pwd) or on minion (salt-call...). That's why we need to sync them on minions to extmods, but also make them loadable on master.

To use that feature, one needs to create '_outputters' dir in file root (to make it syncable) and set outputters_dir master config to that dir (to make it loadable from master). This works perfectly.

Not sure if there is better solution. Usualy custom code in salt is either ran only on minion (states, modules...) or on master (runners), so I've tried to make this change as compliant with the current codebase as I was able to.