add version flag

[romainmenke]

Added a version flag to the cli.

Currently prints : Guetzli 1.0.1

#130

[gtuk]

@romainmenke Maybe another/better solution is to create a header file dynamicly on the built process to read the version and maybe add the commit hash from another file, like other/popular tools do it, sothat its not required to edit the guetzli.cc on every version change

[romainmenke]

@gtuk c++ is not my strong suit, thought I would get the ball rolling by adding the flag to the cli already. Do you have a reference for the method you describe?

[gtuk]

@romainmenke i had something like this http://stackoverflow.com/a/1843783 or this http://m.linuxjournal.com/content/add-auto-incrementing-build-number-your-build-process in mind. Maybe its a good first reference. Let me know if i can help you