CliRunner isolation stdin has name and mode

[jyscao]

• create subclass of io.TextIOWrapper, NamedTextIOWrapper, for PY3
• use NamedTextIOWrapper instance for sys.stdin & sys.stdout inside
  of CliRunner's isolation contextmanager

fixes #1064

A few more notes:

• 'name' & 'mode' are assigned to self.__dict__ as kv-pairs, and not as actual attributes to the self object, because b/c the io.TextIOWrapper object is not writable
• then __getattribute__ is used to get the above 2 pseudo-attributes from self.__dict__
• the bug behavior described stdin click.File has .name attribute when running from CLI, but doesn't when using CliRunner #1064 also exists in PY2, but this PR does not attempt to address that (since Python 2 support will eventually be dropped)
• NamedTextIOWrapper is not applied to sys.stderr, since click does not readily make it available as a click.File parameter type

[kdeldycke]

Can confirm the issue and the fix! 👍

[davidism]

Added changelog and test. Rewrote this to use properties instead of messing with __dict__ and __getattr__. Applied it to all three streams.