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)
```

Note 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.

For example Dialog is disabled by dafault, so we have the "enable" switch in the list.