Add dos2unix plugin

plugins/dos2unix/README.md

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+## Dos2unix filter
+
+This plugin converts CRLF line ending to LF in text files in the repo.
+It is recommended that you have a .gitattributes file that maintains
+the usage of LF endings going forward, for after you have converted your
+repository.
+
+To use the plugin, add
+`--plugin dos2unix`.

plugins/dos2unix/__init__.py

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+def build_filter(args):
+    return Filter(args)
+
+class Filter():
+    def __init__(self, args):
+        pass
+
+    def file_data_filter(self,file_data):
+        file_ctx = file_data['file_ctx']
+        if not file_ctx.isbinary():
+            file_data['data'] = file_data['data'].replace('\r\n', '\n')