Watch Directory CLI Parameter

I wish I could specify an input directory to webc cli tool. That is, instead of doing

```
$ cd $PROJECT_HOME/src
$ $PROJECT_HOME/node_modules/webc/bin/webc -o $PROJECT_HOME/out
```

I would like to be able to do

```
$ $PROJECT_HOME/node_modules/webc/bin/webc -o $PROJECT_HOME/out -i $PROJECT_HOME/src
```

(webc 0.1.2)
