Add support for connecting to DSChief

[ddaws]

Add support for connecting to the Maker DSChief for pulling Governance information from the Maker system including the current hat, proposed slates, and their votes. This is important to support developing and monitoring governance tools.

Some useful links for my future self

• https://docs.makerdao.com/smart-contract-modules/governance-module/chief-detailed-documentation
• https://via.hypothes.is/https://github.com/dapphub/ds-chief/blob/master/src/chief.sol
• https://github.com/makerdao/dai.js/blob/dev/packages/dai-plugin-governance/contracts/addresses/mainnet.json
• https://via.hypothes.is/https://github.com/dapphub/ds-note/blob/master/src/note.sol
• https://solidity.readthedocs.io/en/v0.5.3/layout-of-source-files.html
• https://solidity.readthedocs.io/en/v0.4.21/using-the-compiler.html