Replace HTML chars for shell usage.

def shellfilter(value):
    """Replace HTML chars for shell usage."""
    replacements = {'\\': '\\\\',
                    '`': '\\`',
                    "'": "\\'",
                    '"': '\\"'}
    for search, repl in replacements.items():
        value = value.replace(search, repl)
    return safestring.mark_safe(value)