[Python-Dev] Edits to Metadata 1.2 to add extras (optional dependencies)

[Daniel Holth]

A few more edits at

https://bitbucket.org/dholth/python-peps/diff/pep-0426.txt?diff2=058b058ee3f8&diff1=42ee0afd40ed

rfc822 is nixed in favor of 'parseable by email.parser.Parser with the
right policy'

Encoding is strictly utf-8 with the warning that older revisions have
no defined encoding (keys are still always ASCII)

Extra names are now 'printable ascii except , [ ]' instead of 'valid
Python identifiers'.

Extension names are now 'ascii, with no whitespace or /', with the /
separator between the extension name and the rest of the tag name.

No 'the order doesn't matter' clause yet.