Add static analysis tools

[Nezteb]

Addresses #19

I also added Dialyzer, which seemed like a simple enough value-add. For some of my own notes on the tool, see: https://fly.io/phoenix-files/adding-dialyzer-without-the-pain/

Output of various commands is below.

Credo:

> mix credo
Checking 10 source files ...

Please report incorrect results: https://github.com/rrrene/credo/issues

Analysis took 0.1 seconds (0.07s to load, 0.09s running 55 checks on 10 files)
96 mods/funs, found no issues.

Showing priority issues: ↑ ↗ →  (use `mix credo explain` to explain issues, `mix credo --help` for options).

Sobelow:

> mix sobelow
WARNING: Sobelow cannot find the router. If this is a Phoenix application
please use the `--router` flag to specify the router's location.

##############################################
#                                            #
#          Running Sobelow - v0.13.0         #
#  Created by Griffin Byatt - @griffinbyatt  #
#     NCC Group - https://nccgroup.trust     #
#                                            #
##############################################

... SCAN COMPLETE ...

Dialyzer:

> mix dialyzer
Finding suitable PLTs
Checking PLT...
[:asn1, :castore, :compiler, :crypto, :decimal, :ecto, :eex, :elixir, :jason, :kernel, :logger, :mime, :phoenix, :phoenix_html, :phoenix_pubsub, :phoenix_template, :plug, :plug_crypto, :public_key, :stdlib, :telemetry, :typeid_elixir, :websock, :websock_adapter]
PLT is up to date!
No :ignore_warnings opt specified in mix.exs and default does not exist.

Starting Dialyzer
[
  check_plt: false,
  init_plt: ~c"/Users/noah/Git/typeid-elixir/_build/dev/dialyxir_erlang-26.0.2_elixir-1.15.0_deps-dev.plt",
  files: [~c"/Users/noah/Git/typeid-elixir/_build/dev/lib/typeid_elixir/ebin/Elixir.Inspect.TypeID.beam",
   ~c"/Users/noah/Git/typeid-elixir/_build/dev/lib/typeid_elixir/ebin/Elixir.Jason.Encoder.TypeID.beam",
   ~c"/Users/noah/Git/typeid-elixir/_build/dev/lib/typeid_elixir/ebin/Elixir.Phoenix.HTML.Safe.TypeID.beam",
   ~c"/Users/noah/Git/typeid-elixir/_build/dev/lib/typeid_elixir/ebin/Elixir.Phoenix.Param.TypeID.beam",
   ~c"/Users/noah/Git/typeid-elixir/_build/dev/lib/typeid_elixir/ebin/Elixir.String.Chars.TypeID.beam",
   ...],
  warnings: [:no_opaque, :unmatched_returns, :error_handling, :unknown, ...]
]
Total errors: 0, Skipped: 0, Unnecessary Skips: 0
done in 0m1.47s
done (passed successfully)

[Nezteb]

This is the default file generated by mix sobelow --save-config with a little formatting tweak.

[Nezteb]

This is the default file generated by mix credo gen.config with no alterations on my part.