added cuttlefish_util:conf_get_value/3

[joedevivo]

This function was supposed to be a convenience function for schema writers that wrapped proplists:get_value

It only makes sense to have the arty 3 version

[seancribbs]

Why not simplify this by having the 2-arity version call the 3-arity version with the third argument of undefined.

[joedevivo]

ha, that almost seems "too" correct. will do.

[seancribbs]

Dialyzed this, found some bugs. These are not part of the PR, but should be addressed soon. Ideally rebar would be added to the PLT to resolve the mustache/rebar problems, but I'm not quite sure how to do that without making rebar a dependency.

Unknown functions:
  eunit:test/1
  inti:stop/1
  mustache:render/2
  rebar_rel_utils:get_target_dir/2
  rebar_rel_utils:is_rel_dir/0
  rebar_rel_utils:load_config/2

$ ack "inti:stop" src
src/cuttlefish_escript.erl
105:            inti:stop(1);

[seancribbs]

Disregard the inti:stop error, it is not present on develop.

Rest of this is 👍 to merge.