Improvement: add the "-v/--version" option to the executable

[niconoe]

... to display the running version (I got the idea when trying again the tool in an environment when I manually installed previous versions - just wanted to be sure I was on the last one).

For the implementation, it would be great to DRY and store the version number in one single place (constant in __init__.py, maybe).

[roll]

@niconoe Please take a look at my review - https://github.com/frictionlessdata/FrictionlessDarwinCore/issues/7. For FrictionlessData software we have already sorted out all these non-functional problems like how to store the version etc (see the oki-py link). For every new project, we just copy this skeleton to save developers' time and make community contribution easier.