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.
Adapted check_http2 for use of new ShinkenPlugin class.
Clean: link.py->file_without_ext => link_without_ext->file.py

Better to have the file actually containing the python code be the one with .py extension.
And make the extension-less one links to the .py one.
bump check-http2 version.