Add additional/replacement ginkgomon start/readiness check

[sunjayBhatia]

ginkgomon currently uses the StartCheck string to check the readiness of the process it is monitoring, however for some classes of processes it would be useful to have a readiness check that does not rely on log lines.

Would you be amenable to a change that introduces a StartCheckFunc or some other generalized readiness check the user can specify to determine readiness?

Some open questions/ideas about solutions that are worth discussing:

• what is the relative priority of this new readiness check vs. the existing output check?
• could this replace the existing StartCheck?
  • could we enable users to implement the existing functionality by giving them access to the stdout/stderr buffer from the execed command? this could also be a builtin helper function that wires up the log check for you
• could this instead be a socket or host/port tcp check for servers if a fully generic readiness check is not desired?