Allow to support cmd args flags as config input.

[benjaminch]

More details here #4. This CL includes:

• Update README
• Update example providing a Flask example implementing flags parsing
• Introducing FlagsProvider class dealing with argparse
• Updating vyper logic to take into account flags

[benjaminch]

Hey @admiralobvious, let me know what do you think about this one addressing #4 :) Thanks

[benjaminch]

Abandonned, and recreated here: https://github.com/admiralobvious/vyper/pull/8