Make checkers extensible via entry point

This commit adds an entry point called pydocstyle_styles
which allows checks to be loaded dynamically.

setup.py

@@ -41,5 +41,10 @@
         'console_scripts': [
             'pydocstyle = pydocstyle.cli:main',
         ],
+        'pydocstyle_styles': [
+            'pydocstyle.base = pydocstyle.checkers.style.base',
+            'pydocstyle.numpy = pydocstyle.checkers.style.numpy',
+            'pydocstyle.other = pydocstyle.checkers.style.other',
+        ]
     },
 )

src/pydocstyle/checkers/__init__.py

@@ -1,6 +1,3 @@
 from pydocstyle.checkers.hooks import get_checkers
-# These imports below are required for registering the
-# checkers so that `get_checkers` captures them.
-from pydocstyle.checkers.style import numpy, base, other
 
-__all__ = ('get_checkers', 'numpy', 'base', 'other')
+__all__ = ('get_checkers',)

src/pydocstyle/checkers/hooks.py

@@ -1,4 +1,14 @@
+import pkg_resources
+
+from typing import Set
+
+from pydocstyle.utils import log
+
 __registered_checkers = []
+__loaded_styles = set()  # type: Set[str]
+
+ENTRY_POINT_NAME = 'pydocstyle_styles'
+
 
 def check_for(kind, terminal=False):
     def decorator(f):
@@ -9,5 +19,18 @@ def decorator(f):
     return decorator
 
 
+def _load_styles():
+    for entry_point in pkg_resources.iter_entry_points(ENTRY_POINT_NAME):
+        if entry_point.name not in __loaded_styles:
+            try:
+                entry_point.load()
+            except Exception as error:
+                log.exception("Unable to load plugin %s.\nError occurred: %s",
+                              entry_point.name, error)
+            else:
+                __loaded_styles.add(entry_point.name)
+
+
 def get_checkers():
+    _load_styles()
     return  iter(__registered_checkers)