Smart Merge

Dat smart merge.

How's it work?

Say you include a schema from a dependency that has a mapping like this:

```erlang
%% @doc an amazing configuration setting that I promise 
%% will never not have 10 as a sensible default!
{mapping, "variable.a", "erlang.term", [
    {default, 10},
    {datatype, integer}
]}.
```

So, of course, for your application, 10 is a terrible default. Before you would have to copy this whole mapping into your project's schema and just change the default, but now, you can choose a smart merge:

```erlang
{mapping, "variable.a", "erlang.term", [
    merge,
    {default, 10000}
]}.
```

So wha happen?

well, the actual mapping will have the new default of 10000 *AND* all other attributes of the original mapping, which in this case means `{datatype, integer}` and importantly, the docs from the original schema will be preserved, so your effective mapping would look like this:

```erlang
%% @doc an amazing configuration setting that I promise 
%% will never not have 10 as a sensible default!
{mapping, "variable.a", "erlang.term", [
    {default, 10000},
    {datatype, integer}
]}.
```

Unfortunately, as a consequence of choosing to have `mapping` be a 4-tuple, you'll need to provide the erlang mapping (in this case `"erlang.term"`) again. Whatever you put in the 3rd element here **WILL** override the original erlang mapping.