Allow specifying parameters via command line

Let's extend the parameters optionally defined in `Gruntfile.parameters.js` with the ones optionally specified in the command line.

For instance, the translations grunt task may use the `txPassword` either from `Gruntfile.parameters.js` or from a command line like:
```
grunt translations --txPassword=...
```

If the parameters are specified both in `Gruntfile.parameters.js` and in the command line, the latter is used.