Add support for igniter 0.6

[wingyplus]

I try installing mneme with igniter 0.6 (in my project) and got:

mix deps.get returned exited with code: `1`

** (RuntimeError) Resolving Hex dependencies...
Resolution completed in 0.226s
Because "mneme >= 0.10.2" depends on "igniter ~> 0.3.76 or ~> 0.4.0 or ~> 0.5.0" and "mneme >= 0.10.0 and < 0.10.2" depends on "igniter ~> 0.3.76 or ~> 0.4.0", "mneme >= 0.10.0" requires "igniter ~> 0.3.76 or ~> 0.4.0 or ~> 0.5.0".
And because "your app" depends on "igniter ~> 0.6", "mneme >= 0.10.0" is forbidden.
So, because "your app" depends on "mneme ~> 0.10", version solving failed.
** (Mix) Hex dependency resolution failed

So I add it to the mix.exs to ensure it works with igniter 0.6.

[tcoopman]

@zachallaun any chance you can merge this and push a release?

[linusdm]

I also have an override for igniter in recent projects. Since more and more libraries are using igniter and igniter is evolving, it forces mneme to follow pace.

The dependency conflict could be avoided by making igniter optional like Phoenix LiveView does:

1. Change to {:igniter, "~> 0.5", optional: true} in mix.exs
2. Wrap the Mix.Tasks.Mneme.Install module with if Code.ensure_loaded?(Igniter) and provide a fallback that tells users to install igniter

See how Phoenix LiveView does it: https://github.com/phoenixframework/phoenix_live_view/blob/main/lib/mix/tasks/phoenix_live_view.upgrade.ex