Add default support for crystalline

[notramo]

With the default settings, crystalline doesn't work on a bunch of files. This can be solved by adding a .crystalline_main.cr where the necessary files are imported.

https://github.com/elbywan/crystalline/issues/67

Suggested solution

Add the following to the default shards.yml file:

crystalline:
  main: .crystalline_main.cr

And add this to the .crystalline_main.cr file in the project root:

require "./src/start_server.cr"
require "./db/**"
require "./tasks.cr"