Adding an 'args' parameter to the plugin framework.

PluginExecutor now takes a plugin_args argument, whose value is now passed through as
an argument to the 'run' method of ADAMPlugin.

Added a new example (test) plugin, TakeNPlugin, which shows a simple use of this parameter.
Merge pull request #258 from genomebridge/plugin-args

Adding an 'args' parameter to the plugin framework.