Merge pull request #1020 from takluyver/dollar-formatter Dollar formatter for ! shell calls. This uses a subclass of `FullEvalFormatter` for filling in variables in shell commands like `!echo $spam`, instead of Itpl. I've retained the `{foo}` style standard formatting for complex expressions, so it can be picked up by the more powerful built in parser. `$foo` works for names and attribute access. FullEvalFormatter now always outputs unicode, even if you gave it a bytes string. This avoids problems when you mix 8-bit non-ascii strings and unicode (which e.g. `StringIO.StringIO` suffers from). Closes #822 (test was added in 31ab23f).