Add valid word characters as a configuration option

See sphinx-contrib#119 (comment)

sphinxcontrib/spelling/__init__.py

@@ -54,6 +54,8 @@ def setup(app):
     app.add_config_value('spelling_filters', [], 'env')
     # Set a user-provided list of files to ignore
     app.add_config_value('spelling_exclude_patterns', [], 'env')
+    # Set valid word characters
+    app.add_config_value('spelling_valid_word_characters', 'en_US', 'env')
     # Choose whether or not the misspelled output should be displayed
     # in the terminal
     app.add_config_value('spelling_verbose', True, 'env')

sphinxcontrib/spelling/builder.py

@@ -86,6 +86,7 @@ def init(self):
             word_list_filename=word_list,
             filters=f,
             context_line=self.config.spelling_show_whole_line,
+            valid_word_chars=self.config.spelling_valid_word_characters,
         )
 
     def _load_filter_classes(self, filters):

sphinxcontrib/spelling/checker.py

@@ -21,7 +21,8 @@ class SpellingChecker:
     """
 
     def __init__(self, lang, suggest, word_list_filename,
-                 tokenizer_lang='en_US', filters=None, context_line=False):
+                 tokenizer_lang='en_US', filters=None, context_line=False,
+                 valid_word_chars=None):
         if enchant_import_error is not None:
             raise RuntimeError(
                 'Cannot instantiate SpellingChecker '