Support Overlay Files

The Emboss attribute system was originally designed to allow *overlay* files, which would allow attributes to be specified without editing the original `.emb` file -- this is particularly useful in cases where the `.emb` file is provided by a third party, which is the main reason it has not yet been implemented.

The basic idea is that, given an `.emb` like:

```
struct Foo:
0 [+1] UInt foo_field

enum Bar:
BAR_VALUE = 1
```

You would be able to write a `.emboverlay` file like:

```
[$default (cpp) enum_translation: "kCamelCase"]
[(cpp) namespace: "xyz"]
struct Foo:
[(cpp) name: "Foo_"]
foo_field:
[(cpp) name: "foo_field_"]

enum Bar:
BAR_VALUE
[(cpp) name: "kBarValue_"]
```

The exact details need to be ironed out.