Expose Runner interface to allow custom command runner implementations

[tomhjp]

• Adds a new set of interfaces in a runner package to allow custom implementations for running plugins.
• Adds 3 new client-facing config options
  • RunnerFunc can be supplied to customise how a plugin is run, with *exec.Cmd treated as the spec.
  • SkipHostEnv tells go-plugin not to add the host process' environment variables to the command spec. Closes #104 and closes #163
  • ReattachConfig.ReattachFunc can be supplied to hook into running plugins that are identified by something other than a process ID
• From a server (plugin) point of view; Adds optional environment variables to control the plugin's listener unix sockets
• Adds CmdRunner as the default Runner implementation, maintaining existing functionality and backwards compatibility

See https://github.com/hashicorp/go-secure-stdlib/pull/84 for an example implementation of runner's interfaces.

Not planned (for now):

• netrpc support

[tomhjp]

https://github.com/hashicorp/go-secure-stdlib/pull/84 has an alternative container-based implementation for any reviewers who'd like to compare with CmdRunner.

[tomhjp]

Thanks!