cardano-cli ChainContext impl

[thaddeusdiamond]

For those running a local cardano-node, using Python os.system() calls to shell out to cardano-cli and submit transactions/read from chain could be interesting. Let me know if you would like a pull request for this, I can try to stub one out. Just didn't want to proceed with coding if you had a good reason not to do this default impl in the library.