Crystal support improvements

I'd like to improve Crystal support for up - it currently only works with a ``main.cr`` which is older-school. The current standard practice is to have a ``shard.yml`` which describes the app and targets. Suggestions:

1. I fix #172 - yes there is an official image + can move to ``--release``
2. Add yaml parsing, pull out the (first - to start) target from the ``shard.yml`` file and build that instead of forcing ``main.cr``.

FYI: I made a simple demo app with Kemal that imho should "just work" but doesn't due to issue 2 above -- see https://github.com/ukd1/up-crystal-demo - result after running ``up`` on it is this (as it doesn't detect crystal):

@tj thoughts?