v1.5.0 - Telos support + fixes

• Re-factored EOSMixin, EOSLoader, and EOSManager to use the new static attributes chain, chain_type, and chain_coin - allowing for painless
  sub-classing for EOS forks.
  • Moved URL generation code out of EOSMixin.url into EOSMixin._make_url to allow for other methods to generate URLs
  • Added replace_eos method for replacing the Cleos instance, e.g. when a new RPC URL needs to be used
  • Adjusted many methods to use chain chain_type and chain_coin instead of hard coded 'eos' / 'EOS' to make sub-classing for forks easier
  • Added current_rpc attribute, enabling the ability to check what RPC is currently being used by the Cleos instance
• Created Telos handler and enabled it by default. Mostly stub code which just sets the chain/chain_type/chain_coin so the EOS handler uses it correctly
• Improved the EOSLoader list_txs so that it can change the RPC for different tokens if needed
• Added enabled=True to all handler __init__ files, to prevent handlers trying to load disabled coins and erroring
• Changed dotenv.read_dotenv to dotenv.load_env (apparently a different dotenv package is being used now?)