Enh: New ShinkenPlugin base class.

+ Moved STATES to own module for easy reuse.
+ custom PluginParser for customized exit().
+ better add_threshold() functions.
+ helpers with parsing methods for "size" amounts (MB|KB|..).

+ included argparse if one doesn't want to install it from system packages.
Fix: PerfData: set values to '' only if None.
Change check_http2 to use new ShinkenPlugin class.