Dump runtime features with --list-runtime-features

This is how the output looks like currently:

```
$ ./xwalk --list-runtime-features
Runtime features:
  --disable-raw-sockets (disable JavaScript support for using TCP and UDP sockets)
  --disable-device-capabilities (disable JavaScript support for peeking at device capabilities)
  --enable-dialog (enable JavaScript support to create open/save native dialogs)
```

Not that the code takes into account what is the default state of the feature and prints only the command line switch that can actually change this state.