Add command-line help

[giovannipizzi]

To print:

• basic information on what are the possible command line options
• print some help when wannier is run without options
• add a --dry-run option to validate the input file
• add a -v and/or --version option to print the code version and if possible the compilation flags

[jryates]

I've got a prototype for this, I just want to check one thing. At the moment if you run wannier90 with no arguments it tries to read wannier.win. I think that was a mistake - and it would be better if the wannier90 printed usage information and quit. Do you agree?

[mostofi]

@jryates - I agree.

[giovannipizzi]

I agree as well!