Based on `comments.toml` but keyed by file extension instead of language
name.

Generated using the following two scripts with some additional manual
modification:

convert_file_extension_keys_toml.py:

    #!/usr/bin/env python3

    import toml

    comments = toml.load('comments.toml')

    by_extension = {}

    for language, attributes in comments.items():
        if 'extensions' in attributes:
            value = attributes
            extensions = value.pop('extensions')
            value['name'] = language

            for extension in extensions:
                by_extension[extension] = value

    with open('comments_by_extension.toml', 'w') as f:
        toml.dump(by_extension, f)

sort_file_extensions.py:

    #!/usr/bin/env python3

    from collections import OrderedDict

    import toml

    comments = toml.load('comments_by_extension.toml')
    comments = OrderedDict(sorted(comments.items()))

    with open('comments_by_extension.toml', 'w') as f:
        toml.dump(comments, f)

A database of comment markers in many languages. Used the list of
languages on:

http://rosettacode.org/wiki/Comments

Discovered the comment marker syntax from the examples on that page.

The file extensions were added manually and informed by language
homepages, GitHub, and Wikipedia.