Mode specific and custom wrap indents

Currently Ace supports three wrapping behaviors:

1. wrap indent disabled:
```
This line wraps with no
indentation at all.
```

2. text mode, wrap indent enabled:

```
This line wraps with indentation
matching the its initial indent.
```

3. code mode, wrap indent enabled

``` javacript
stdout.write("This line wraps with indentation
a fixed amount beyond its initial indent.")
```

[These behaviors are hard-coded](https://github.com/ajaxorg/ace/blob/a505922a01395ab77a878c08da06eb09b7822d23/lib/ace/edit_session.js#L1781), without any way for a Mode to hook in custom behavior to suit the particular conventions of its file type, nor a means to add configuration options.

Some desirable alternate behaviors:

- In plain text, wrap indentation that aligns with the content on the first line gives the most legibilty. I fact,
CommonMark specifies this for *hard* wrapping of list content because
``` markdown
1. When this line is wrapped, it should line up
with the word "When".
```

- In code, some prefer *semantic* indentation, such as wrapping [wherein arguments are aligned](https://packagecontrol.io/packages/Align%20Arguments)
``` javascript
someLongFunction(argument1, argument2, argument3
argument4, argument5);
```

I propose that `getWrapIndent()` be customizable by Mode. I'm even willing to do the implementation. But first, would such changes be approved by the project maintainers?