Add support for post-processing of LSP messages

Messages which are returned from LSP servers or tsserver can be multi-line messages which are pretty long, and don't look so good when echoed on a single truncated line. One example is a tsserver message for a missing attribute in an object literal. Here's an example from a codebase I work on.

```
Type '{ type: "Text"; min_length: number; max_length: number; }' is not assignable to type 'FormField'.
Type '{ type: "Text"; min_length: number; max_length: number; }' is not assignable to type 'TextField'.
Property 'required' is missing in type '{ type: "Text"; min_length: number; max_length: number;
```

The truncated message I see is something like the following.

```
Type '{ type: "Text"; min_length: number; max_length: number; }' is not assignable to type 'FormField'. Type ...e 'TextField'. Property 'required' is missing in type '{ type: "Text"; min_length: number; max_length: number; }'.
```

It would be nicer to display the last line of output for the short message instead, and reveal the rest with `:ALEDetail`.

```
Property 'required' is missing in type '{ type: "Text"; min_length: number; max_length: number;
```

I propose adding an optional callback to linter definitions for LSP or tsserver linters which receive `(buffer, loclist)` as their input. Then the callback can return a modified `loclist` value. Then it will be possible to generate better message to echo, ignore certain messages, or generally modify the list of problems any way that makes sense.