v0.9.0

This is a major update to Shoehorn that includes breaking changes:

• The :init configuration option only supports applications now. MFAs are no
  longer supported and moved to runtime.exs or an Application.start
  callback.
• References to use Shoehorn.Handler need to be updated to
  @behaviour Shoehorn.Handler. This may require implementing additional
  functions.
• Elixir 1.9 is no longer supported. Please update to Elixir 1.10 or later.

The main update to Shoehorn is to move all application startup to the boot
script. This noticeably improves boot time on many Nerves platforms due to boot
scripts being able to load files without traversing the entire Erlang module
path list. These traversals are amazingly slow (sometimes seconds) due to a
combination of SquashFS slowness in this area and slow overall IO.

Using boot scripts to load all applications has some important improvements in
addition to performance:

• Application start order is deterministic and computed at compile-time. If you
  want to see the order, take a look at the end of the shoehorn.script in your
  release directory.
• Shoehorn alphabetizes the start of applications that could be ordered
  arbitrarily. This minimizes changes in start ordering when dependencies are
  added or removed.
• It enables experimental features like providing additional dependencies (using
  the :extra_dependencies configuration key) or hinting that dependencies get
  started as late as possible (the :last configuration key)
• You can remove the :app configuration key from your Shoehorn configuration.
  It's no longer needed.

Aside from the change from a macro to a behaviour and possibly needing to
implement callback functions, Shoehorn.Handler implementations work the same
as before.