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]

Hello @admiralobvious,

Did you get a chance to look to this PR? I made couple improvements and it works perfectly in production on couple projects already.

Please let me know what you think.

Thanks ! ;)

[alexferl]

LGTM, thank you! Sorry for the delay, I was on vacation.

[benjaminch]

Thanks a lot, no worries ;)