CLI command for current version

[florimondmanca]

Is your feature request related to a problem? Please describe.
There should be an easier way to retrieve the current Bocadillo than writing an inline Python script or entering the shell (as described in the Installation instructions).

Describe the solution you'd like

Let's get boca a version command/option:

boca version
boca --version
boca -v
boca -V

All these should show the currently installed, e.g. "0.6.1".