Use raw output in `whereami`

To prevent things like XML tags in code from messing up output.
Update CodeFormatter to use PhpConsoleHighlighter.

Makes `show` output match `whereami`. Also, it's prettier :)
`show` command no longer needs formatted signature.

… except as fallback when source code is unavailable. 
Suppress deprecation notice for Symfony TableHelper.

We'll deal with it for reals when Console component hits 3.0.

Since everyone does (or should) use `E_ALL` in development, `E_USER_DEPRECATED` is disruptive enough that it's essentially a backwards compatibility break. This is not cool. My users should not have to choose between requiring an upgrade on a point release, and not getting any more bug fixes or new features from PsySH.

Fixes #132