Make it explicit but optional and mitigate conflicts

lib/elixlsx/xml_templates.ex

@@ -175,7 +175,7 @@ defmodule Elixlsx.XMLTemplates do
 
   defp split_into_content_style(cell, _wci), do: {cell, 0, nil}
 
-  if Code.ensure_loaded?(Decimal) do
+  if Code.ensure_loaded?(Decimal) and function_exported?(Decimal, :to_string, 2) do
     defp get_content_type_value(%Decimal{} = content, _wci) do
       {"n", Decimal.to_string(content, :normal)}
     end

mix.exs

@@ -24,6 +24,7 @@ defmodule Elixlsx.Mixfile do
 
   defp deps do
     [
+      {:decimal, "~> 1.6 or ~> 2.0", optional: true},
       {:excheck, "~> 0.5", only: :test},
       {:triq, "~> 1.0", only: :test},
       {:credo, "~> 0.5", only: [:dev, :test]},

mix.lock

@@ -1,6 +1,7 @@
 %{
   "bunt": {:hex, :bunt, "0.1.6", "5d95a6882f73f3b9969fdfd1953798046664e6f77ec4e486e6fafc7caad97c6f", [:mix], [], "hexpm", "4fb7b2f7b04af13cf210b132f8d10db52d4a57d36cb974e8025d7fdb12ca97fc"},
   "credo": {:hex, :credo, "0.5.3", "0c405b36e7651245a8ed63c09e2d52c2e2b89b6d02b1570c4d611e0fcbecf4a2", [:mix], [{:bunt, "~> 0.1.6", [hex: :bunt, repo: "hexpm", optional: false]}], "hexpm", "90bca5180fe64c47343969ad3e32bf1edc18d929689e8c00c810655a7a391427"},
+  "decimal": {:hex, :decimal, "2.0.0", "a78296e617b0f5dd4c6caf57c714431347912ffb1d0842e998e9792b5642d697", [:mix], [], "hexpm", "34666e9c55dea81013e77d9d87370fe6cb6291d1ef32f46a1600230b1d44f577"},
   "dialyxir": {:hex, :dialyxir, "0.5.1", "b331b091720fd93e878137add264bac4f644e1ddae07a70bf7062c7862c4b952", [:mix], [], "hexpm", "6c32a70ed5d452c6650916555b1f96c79af5fc4bf286997f8b15f213de786f73"},
   "earmark": {:hex, :earmark, "1.3.1", "73812f447f7a42358d3ba79283cfa3075a7580a3a2ed457616d6517ac3738cb9", [:mix], [], "hexpm", "000aaeff08919e95e7aea13e4af7b2b9734577b3e6a7c50ee31ee88cab6ec4fb"},
   "earmark_parser": {:hex, :earmark_parser, "1.4.12", "b245e875ec0a311a342320da0551da407d9d2b65d98f7a9597ae078615af3449", [:mix], [], "hexpm", "711e2cc4d64abb7d566d43f54b78f7dc129308a63bc103fbd88550d2174b3160"},