feat(config): Add support to ignore paths on per-rule basis

Example of configuration to use this feature:

    # For all rules
    ignore: |
      *.dont-lint-me.yaml
      /bin/
      !/bin/*.lint-me-anyway.yaml

    rules:
      key-duplicates:
        ignore: |
          generated
          *.template.yaml
      trailing-spaces:
        ignore: |
          *.ignore-trailing-spaces.yaml
          /ascii-art/*

Closes #43.

setup.py

@@ -46,7 +46,7 @@ setup(
     entry_points={'console_scripts': ['yamllint=yamllint.cli:run']},
     package_data={'yamllint': ['conf/*.yaml'],
                   'tests': ['yaml-1.2-spec-examples/*']},
-    install_requires=['pyyaml'],
+    install_requires=['pathspec', 'pyyaml'],
     tests_require=['nose'],
     test_suite='nose.collector',
 )