Add --combine option

Alex, thanks for an awesome project!

From the CLI, I'd like the ability to control the output to the namespace array.  Currently, the namespace element is based on the file name.  If I have a folder structure like:

    en
    |-- one.json
    |-- two.json

the resulting, generated namespace (i18n) array looks like:

```javascript
window.i18n['one']
window.i18n['two']
```
I'd like the option to combine multiple files (recursively found) in a directory into a single namespace element per language.  The pull request implements this behavior by adding a **'-c'** or **'--combine'** option such as:

    $ messageformat -l en -c en [input] [output]

At a higher level, I'm trying to achieve (using an AngularJS service and filter) a result similar to the link below but with multiple input files:

https://gist.github.com/iammerrick/5518936

What do you think?  Acceptable?

Thanks again!