Deprecate everything we're removing in 1.0

This adds deprecation warnings to all the things we'll be removing when
1.0 is released. We can probably release this as version 0.99 to make it
clear that this is just a release that adds deprecations before 1.0.

lib/benchee.ex

@@ -44,6 +44,11 @@ for {module, moduledoc} <- [{Benchee, elixir_doc}, {:benchee, erlang_doc}] do
     end
 
     def run(config, jobs) when is_map(jobs) do
+      IO.puts("""
+      Passing configuration as the first argument to `run/2` is deprecated.
+      Please see the documentation for `run/2` for updated usage instructions.
+      """)
+
       # pre 0.6.0 way of passing in the config first and as a map
       do_run(jobs, config)
     end

lib/benchee/configuration.ex

@@ -347,6 +347,14 @@ defmodule Benchee.Configuration do
 
   defp formatter_configuration_from_options(config, module, legacy_option_key) do
     if Map.has_key?(config.formatter_options, legacy_option_key) do
+      IO.puts("""
+
+      Using `:formatter_options` to configure formatters is now deprecated.
+      Please see the documentation for `Benchee.Configuration.init/1` for
+      current usage instructions.
+
+      """)
+
       {module, config.formatter_options[legacy_option_key]}
     else
       module

lib/benchee/formatter.ex

@@ -73,7 +73,20 @@ defmodule Benchee.Formatter do
     {formatter, @default_opts}
   end
 
-  defp normalize_module_configuration(formatter), do: formatter
+  defp normalize_module_configuration(formatter) when is_function(formatter, 1), do: formatter
+
+  defp normalize_module_configuration(formatter) do
+    IO.puts("""
+
+    All formaters that are not modules implementing the `Benchee.Formatter`
+    behaviour or functions with an arity of 1 are deprecated.
+    Please see the documentation for `Benchee.Configuration.init/1` for
+    current usage instructions.
+
+    """)
+
+    formatter
+  end
 
   defp is_formatter_module?({formatter, _options}) when is_atom(formatter) do
     module_attributes = formatter.module_info(:attributes)