keepAlive option

Is there away to keep a consuming stream alive even if one of it's feeders sends an end of stream marker?... In my use case I'm dealing with multiple child process stdout streams that will die if the process dies.

for e.g

```
var _ = require("highland");

var mainStr = _().map(function(dat){
return "[prefix] - " + dat;
});
mainStr.pipe(process.stdout);

str1 = _();
str1.pipe(mainStr);
str2 = _();
srt2.pipe(mainStr);

str1.write("here is a message 1");
str2.write("here is a message 2");
str1.write(_.nil);
str2.write("this message will cause an error to be thrown");

// results in error thrown:
// Error: Can not write to stream after nil
```

Is there a way to keep the stream alive, but still clean up after itself when feeders die?
for e.g a `keepAlive` method would be useful.

```
var mainStr = _().keepAlive().map(function(dat){
return "[prefix] - " + dat;
});
```

Start by tracing Highland's handling of _.nil when multiple feeder streams are piped into one consuming stream. Define the intended behavior when one feeder ends, when later feeders write, and when all feeders have ended; then add tests covering the issue's str1/str2 example and cleanup behavior.

Written by the indexing model from the issue text.