Custom operators

RxJS [allows](https://github.com/ReactiveX/RxJava/wiki/Implementing-Your-Own-Operators) to implement your own custom operators.

What about Highland? I don't see any documentation on this. Is it conventional? If not, why?
Working with complex data like Vinyl objects in Gulp streams it would be great to have such option.

Example case, a lot of boilerplate and repeating code:

``` javascript
// === Gulp is globbing over JSON files ===
...
// Update some field in JSON
.map(file => {
let json = file.contents.toString();
let js = JSON.parse(json);
// update obj here...
let str = JSON.stringify(obj, null, 2);
file.contents = new Buffer(str);
return file;
})
...
// Filter over another field in JSON
.filter(file => {
let json = file.contents.toString(); // this mess again ^_^
let js = JSON.parse(json);
// evaluate cond here...
return cond;
})
```

Data is too complex above and operators are weak. It probably may be like this:

``` javascript
// === Gulp is globbing over JSON files ===
...
.pipe(Highland())
.asObject()
// Update some field in JSON
.map(obj => {
// update obj here...
return obj;
})
...
// Filter over another field in JSON
.filter(obj => {
// evaluate cond here...
return cond;
})
.asVinyl()
```

So I basically add `asObject` and `asVinyl` operators which convert Vinyl object with JSON content into JS object and vice versa and use existing operators like `map` and `filter` in their full power.

Start by reviewing Highland's existing operator API around map and filter, then compare the proposed asObject/asVinyl conversions with the Vinyl and JSON example. Define the intended custom-operator behavior and what would count as done; the issue names no files or tests, so repository exploration is required.

Written by the indexing model from the issue text.