Support STATE file

[bfischetti]

We need to be able to receive a state file as parameter where we will persist the state of the stream periodically.

As specified in the doc:

State
The JSON encoded state is used to persist information between invocations of a Tap.

A Tap that wishes to persist state should periodically write STATE messages to stdout as it processes the stream, and should expect the file named by the --state STATE argument to have the same format as the value of the STATE messages it emits.

A common use case of state is to record the spot in the stream where the last invocation left off. If the Tap is invoked without a --state STATE argument, it should start at the beginning of the stream or at some appropriate default position. If it is invoked with a --state STATE argument it should read in the state file and start from the corresponding position in the stream.

Sub-task of #1