KAZOO-2712: callflow module for after-bridge actions

I think calling a function that validates the PostBridgeData, based on Action, would be more appropriate. Since 'park' and 'hangup' would need to be boolean and 'transfer' an extension.

An alternative is the have the "data" object be `{"park|hangup":true}` or `{"transfer":"1000"}` then do
```
{[PostBridgeData], [Action]} = wh_json:get_values(Data),
handle(Call, Action, PostBridgeData).

handle(Call, <<"park">>, ShouldPark) ->
    set(Call, <<"Park-After-Pickup">>, wh_util:is_true(ShouldPark));
handle(Call, <<"hangup">>, ShouldHangup) ->
    set(Call, <<"Hangup-After-Park">>, wh_util:is_true(ShouldHangup));
handle(Call, <<"transfer">>, Extension) ->
    set(Call, <<"Transfer-After-Park">>, Extension);
handle(_Call, _Action, _) ->
    lager:debug("unhandled action ~s", [_Action]).

set(Call, Key, Value) ->
    whapps_call_command:set(wh_json:from_list([{Key, Value}]), 'undefined', Call).
```

Untested, but that could help be more explicit about what's possible in these various scenarios.

Questions? Thoughts?