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=...
Update README.md

Let's describe how to use options, and tell which are the options for
the 'translate' task.
Fix sentence in build/README.md
Merge pull request #1495 from mlocati/grunt-allow-commandline-params

Allow specifying parameters via command line