Allow specifying line ending style to Unicode filter

[todofixthis]

When initialising the Unicode filter, the user should have the ability to specify which line endings to use:

# Unix line endings (default)
>>> f.Unicode(convert_newlines='\n').apply('Foo\nBar\rBaz\r\n')
'Foo\nBar\nBaz\n'

# Windows line endings
>>> f.Unicode(convert_newlines='\r\n').apply('Foo\nBar\rBaz\r\n')
'Foo\r\nBar\r\nBaz\r\n'

# Custom line endings
>>> f.Unicode(convert_newlines='|').apply('Foo\nBar\rBaz\r\n')
'Foo|Bar|Baz'

# Line endings unmodified
>>> f.Unicode(convert_newlines=False).apply('Foo\nBar\rBaz\r\n')
'Foo\nBar\rBaz\r\n'

Note that this change potentially conflicts with the normalize argument, so we'll probably need to make a couple of additional changes to support this new functionality:

• Add a remove_unprintables argument.
• Rename normalize to normalize_unicode.
• Deprecate the normalize argument.

[todofixthis]

Might make more sense to create 3 new filters (StripUnprintables, NormalizeUnicode, ConvertNewlines), and then convert the Unicode filter into a macro. Just sayin'... 😸