Add support for --inline-sourcemap

This changeset add support for the `--inline-sourcemap` option. When specified, it adds the source-map as a base64-encoded string at the end of a compiled file `foo.js`, instead of writing it in a separate `foo.map` file.

Notably, this enables the use of traceur's source-maps with browserify, that only supports merging inline source-maps (that's my initial use case). Moreover, inline source-maps are much easier to manipulate since a single stream contains all the information (compiled JS + source-map). If we can at some point output to stdout, we could have fancy all-in-one command-lines:

```
cat es6-foo.js | traceur | do-smth-else > es5-foo.bar
```

Where `do-smth-else` do more transformations, including on the source map; without any intermediary disk file.

This changeset is not breaking the API.

Seems to be related with #892.