Suspend hidden outputs
Add outputOptions function
Add suspended option to Observer
Add tests for suspended observers
Change resume behavior for Observer

Eliminate multiple runs when resumed multiple times
Observer: .flushCallbacks to .invalidateCallbacks
Clarify suspend description
Fix tests
Send output hidden state on init
Stop extra update message from occurring on startup
Run manageHiddenOutputs on app init
Call manageHiddenOutputs when suspendWhenHidden is set
Add documentation for suspendWhenHidden option
Better checks for hidden output objects
Don't allow another flush if currently in one
Call manageHiddenOutputs after timer callbacks
manageHiddenOutputs: check that output object exists
Re-roxygenize
Merge pull request #97 from wch/suspend-hidden

Suspend hidden outputs. Fixes #24
Remove leftover testing string